doc/src/sgml/func/func-string.sgml - master - balmaster/postgres

doc/src/sgml/func/func-string.sgml
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Tom Lane
Doc: split functions-posix-regexp section into multiple subsections.
28 мар 2026, 00:41
28 мар 2026, 00:4100c025a
  <sect1 id="functions-string">
   <title>String Functions and Operators</title>

   <para>
    This section describes functions and operators for examining and
    manipulating string values.  Strings in this context include values
    of the types <type>character</type>, <type>character varying</type>,
    and <type>text</type>.  Except where noted, these functions and operators
    are declared to accept and return type <type>text</type>.  They will
    interchangeably accept <type>character varying</type> arguments.
    Values of type <type>character</type> will be converted
    to <type>text</type> before the function or operator is applied, resulting
    in stripping any trailing spaces in the <type>character</type> value.
   </para>

   <para>
    <acronym>SQL</acronym> defines some string functions that use
    key words, rather than commas, to separate
    arguments.  Details are in
    <xref linkend="functions-string-sql"/>.
    <productname>PostgreSQL</productname> also provides versions of these functions
    that use the regular function invocation syntax
    (see <xref linkend="functions-string-other"/>).
   </para>

   <note>
    <para>
     The string concatenation operator (<literal>||</literal>) will accept
     non-string input, so long as at least one input is of string type, as shown
     in <xref linkend="functions-string-sql"/>.  For other cases, inserting an
     explicit coercion to <type>text</type> can be used to have non-string input
     accepted.
    </para>
   </note>

   <table id="functions-string-sql">
    <title><acronym>SQL</acronym> String Functions and Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function/Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>character string</primary>
         <secondary>concatenation</secondary>
        </indexterm>
        <type>text</type> <literal>||</literal> <type>text</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Concatenates the two strings.
       </para>
       <para>
        <literal>'Post' || 'greSQL'</literal>
        <returnvalue>PostgreSQL</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>||</literal> <type>anynonarray</type>
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <type>anynonarray</type> <literal>||</literal> <type>text</type>
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the non-string input to text, then concatenates the two
        strings.  (The non-string input cannot be of an array type, because
        that would create ambiguity with the array <literal>||</literal>
        operators.  If you want to concatenate an array's text equivalent,
        cast it to <type>text</type> explicitly.)
       </para>
       <para>
        <literal>'Value: ' || 42</literal>
        <returnvalue>Value: 42</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>btrim</primary>
        </indexterm>
        <function>btrim</function> ( <parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters
        in <parameter>characters</parameter> (a space by default)
        from the start and end of <parameter>string</parameter>.
       </para>
       <para>
        <literal>btrim('xyxtrimyyx', 'xyz')</literal>
        <returnvalue>trim</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>normalized</primary>
        </indexterm>
        <indexterm>
         <primary>Unicode normalization</primary>
        </indexterm>
         <type>text</type> <literal>IS</literal> <optional><literal>NOT</literal></optional> <optional><parameter>form</parameter></optional> <literal>NORMALIZED</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Checks whether the string is in the specified Unicode normalization
        form.  The optional <parameter>form</parameter> key word specifies the
        form: <literal>NFC</literal> (the default), <literal>NFD</literal>,
        <literal>NFKC</literal>, or <literal>NFKD</literal>.  This expression can
        only be used when the server encoding is <literal>UTF8</literal>.  Note
        that checking for normalization using this expression is often faster
        than normalizing possibly already normalized strings.
       </para>
       <para>
        <literal>U&amp;'\0061\0308bc' IS NFD NORMALIZED</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>bit_length</primary>
        </indexterm>
        <function>bit_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bits in the string (8
        times the <function>octet_length</function>).
       </para>
       <para>
        <literal>bit_length('jose')</literal>
        <returnvalue>32</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>char_length</primary>
        </indexterm>
        <indexterm>
         <primary>character string</primary>
         <secondary>length</secondary>
        </indexterm>
        <indexterm>
         <primary>length</primary>
         <secondary sortas="character string">of a character string</secondary>
         <see>character string, length</see>
        </indexterm>
        <function>char_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para role="func_signature">
        <indexterm>
         <primary>character_length</primary>
        </indexterm>
        <function>character_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of characters in the string.
       </para>
       <para>
        <literal>char_length('jos&eacute;')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm id="function-lower">
         <primary>lower</primary>
        </indexterm>
        <function>lower</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the string to all lower case, according to the rules of the
        database's locale.
       </para>
       <para>
        <literal>lower('TOM')</literal>
        <returnvalue>tom</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>lpad</primary>
        </indexterm>
        <function>lpad</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>integer</type>
        <optional>, <parameter>fill</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extends the <parameter>string</parameter> to length
        <parameter>length</parameter> by prepending the characters
        <parameter>fill</parameter> (a space by default).  If the
        <parameter>string</parameter> is already longer than
        <parameter>length</parameter> then it is truncated (on the right).
       </para>
       <para>
        <literal>lpad('hi', 5, 'xy')</literal>
        <returnvalue>xyxhi</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ltrim</primary>
        </indexterm>
        <function>ltrim</function> ( <parameter>string</parameter> <type>text</type>
        <optional>, <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters in
        <parameter>characters</parameter> (a space by default) from the start of
        <parameter>string</parameter>.
       </para>
       <para>
        <literal>ltrim('zzzytest', 'xyz')</literal>
        <returnvalue>test</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm id="function-normalize">
         <primary>normalize</primary>
        </indexterm>
        <indexterm>
         <primary>Unicode normalization</primary>
        </indexterm>
        <function>normalize</function> ( <type>text</type>
        <optional>, <parameter>form</parameter> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the string to the specified Unicode
        normalization form.  The optional <parameter>form</parameter> key word
        specifies the form: <literal>NFC</literal> (the default),
        <literal>NFD</literal>, <literal>NFKC</literal>, or
        <literal>NFKD</literal>.  This function can only be used when the
        server encoding is <literal>UTF8</literal>.
       </para>
       <para>
        <literal>normalize(U&amp;'\0061\0308bc', NFC)</literal>
        <returnvalue>U&amp;'\00E4bc'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <function>octet_length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bytes in the string.
       </para>
       <para>
        <literal>octet_length('jos&eacute;')</literal>
        <returnvalue>5</returnvalue> (if server encoding is UTF8)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>octet_length</primary>
        </indexterm>
        <function>octet_length</function> ( <type>character</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns number of bytes in the string.  Since this version of the
        function accepts type <type>character</type> directly, it will not
        strip trailing spaces.
       </para>
       <para>
        <literal>octet_length('abc '::character(4))</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>overlay</primary>
        </indexterm>
        <function>overlay</function> ( <parameter>string</parameter> <type>text</type> <literal>PLACING</literal> <parameter>newsubstring</parameter> <type>text</type> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces the substring of <parameter>string</parameter> that starts at
        the <parameter>start</parameter>'th character and extends
        for <parameter>count</parameter> characters
        with <parameter>newsubstring</parameter>.
        If <parameter>count</parameter> is omitted, it defaults to the length
        of <parameter>newsubstring</parameter>.
       </para>
       <para>
        <literal>overlay('Txxxxas' PLACING 'hom' FROM 2 FOR 4)</literal>
        <returnvalue>Thomas</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>position</primary>
        </indexterm>
        <function>position</function> ( <parameter>substring</parameter> <type>text</type> <literal>IN</literal> <parameter>string</parameter> <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns first starting index of the specified
        <parameter>substring</parameter> within
        <parameter>string</parameter>, or zero if it's not present.
       </para>
       <para>
        <literal>position('om' IN 'Thomas')</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>rpad</primary>
        </indexterm>
        <function>rpad</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>length</parameter> <type>integer</type>
        <optional>, <parameter>fill</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extends the <parameter>string</parameter> to length
        <parameter>length</parameter> by appending the characters
        <parameter>fill</parameter> (a space by default).  If the
        <parameter>string</parameter> is already longer than
        <parameter>length</parameter> then it is truncated.
       </para>
       <para>
        <literal>rpad('hi', 5, 'xy')</literal>
        <returnvalue>hixyx</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>rtrim</primary>
        </indexterm>
        <function>rtrim</function> ( <parameter>string</parameter> <type>text</type>
         <optional>, <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters in
        <parameter>characters</parameter> (a space by default) from the end of
        <parameter>string</parameter>.
       </para>
       <para>
        <literal>rtrim('testxxzx', 'xyz')</literal>
        <returnvalue>test</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substring</primary>
        </indexterm>
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <optional> <literal>FROM</literal> <parameter>start</parameter> <type>integer</type> </optional> <optional> <literal>FOR</literal> <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>string</parameter> starting at
        the <parameter>start</parameter>'th character if that is specified,
        and stopping after <parameter>count</parameter> characters if that is
        specified.  Provide at least one of <parameter>start</parameter>
        and <parameter>count</parameter>.
       </para>
       <para>
        <literal>substring('Thomas' FROM 2 FOR 3)</literal>
        <returnvalue>hom</returnvalue>
       </para>
       <para>
        <literal>substring('Thomas' FROM 3)</literal>
        <returnvalue>omas</returnvalue>
       </para>
       <para>
        <literal>substring('Thomas' FOR 2)</literal>
        <returnvalue>Th</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <literal>FROM</literal> <parameter>pattern</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts the first substring matching POSIX regular expression; see
        <xref linkend="functions-posix-substring"/>.
       </para>
       <para>
        <literal>substring('Thomas' FROM '...$')</literal>
        <returnvalue>mas</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <literal>SIMILAR</literal> <parameter>pattern</parameter> <type>text</type> <literal>ESCAPE</literal> <parameter>escape</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>substring</function> ( <parameter>string</parameter> <type>text</type> <literal>FROM</literal> <parameter>pattern</parameter> <type>text</type> <literal>FOR</literal> <parameter>escape</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts the first substring matching <acronym>SQL</acronym> regular expression;
        see <xref linkend="functions-similarto-regexp"/>.  The first form has
        been specified since SQL:2003; the second form was only in SQL:1999
        and should be considered obsolete.
       </para>
       <para>
        <literal>substring('Thomas' SIMILAR '%#"o_a#"_' ESCAPE '#')</literal>
        <returnvalue>oma</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>trim</primary>
        </indexterm>
        <function>trim</function> ( <optional> <literal>LEADING</literal> | <literal>TRAILING</literal> | <literal>BOTH</literal> </optional>
        <optional> <parameter>characters</parameter> <type>text</type> </optional> <literal>FROM</literal>
        <parameter>string</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Removes the longest string containing only characters in
        <parameter>characters</parameter> (a space by default) from the
        start, end, or both ends (<literal>BOTH</literal> is the default)
        of <parameter>string</parameter>.
       </para>
       <para>
        <literal>trim(both 'xyz' from 'yxTomxx')</literal>
        <returnvalue>Tom</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>trim</function> ( <optional> <literal>LEADING</literal> | <literal>TRAILING</literal> | <literal>BOTH</literal> </optional> <optional> <literal>FROM</literal> </optional>
        <parameter>string</parameter> <type>text</type> <optional>,
        <parameter>characters</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        This is a non-standard syntax for <function>trim()</function>.
       </para>
       <para>
        <literal>trim(both from 'yxTomxx', 'xyz')</literal>
        <returnvalue>Tom</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>unicode_assigned</primary>
        </indexterm>
        <function>unicode_assigned</function> ( <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns <literal>true</literal> if all characters in the string are
        assigned Unicode codepoints; <literal>false</literal> otherwise. This
        function can only be used when the server encoding is
        <literal>UTF8</literal>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>upper</primary>
        </indexterm>
        <function>upper</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the string to all upper case, according to the rules of the
        database's locale.
       </para>
       <para>
        <literal>upper('tom')</literal>
        <returnvalue>TOM</returnvalue>
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Additional string manipulation functions and operators are available
    and are listed in <xref linkend="functions-string-other"/>.  (Some of
    these are used internally to implement
    the <acronym>SQL</acronym>-standard string functions listed in
    <xref linkend="functions-string-sql"/>.)
    There are also pattern-matching operators, which are described in
    <xref linkend="functions-matching"/>, and operators for full-text
    search, which are described in <xref linkend="textsearch"/>.
   </para>

   <table id="functions-string-other">
    <title>Other String Functions and Operators</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function/Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>character string</primary>
         <secondary>prefix test</secondary>
        </indexterm>
        <type>text</type> <literal>^@</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if the first string starts with the second string
        (equivalent to the <function>starts_with()</function> function).
       </para>
       <para>
        <literal>'alphabet' ^@ 'alph'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>ascii</primary>
        </indexterm>
        <function>ascii</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the numeric code of the first character of the argument.
        In <acronym>UTF8</acronym> encoding, returns the Unicode code point
        of the character.  In other multibyte encodings, the argument must
        be an <acronym>ASCII</acronym> character.
       </para>
       <para>
        <literal>ascii('x')</literal>
        <returnvalue>120</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>chr</primary>
        </indexterm>
        <function>chr</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the character with the given code. In <acronym>UTF8</acronym>
        encoding the argument is treated as a Unicode code point. In other
        multibyte encodings the argument must designate
        an <acronym>ASCII</acronym> character.  <literal>chr(0)</literal> is
        disallowed because text data types cannot store that character.
      </para>
      <para>
        <literal>chr(65)</literal>
        <returnvalue>A</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>concat</primary>
        </indexterm>
        <function>concat</function> ( <parameter>val1</parameter> <type>"any"</type>
         <optional>, <parameter>val2</parameter> <type>"any"</type> <optional>, ...</optional> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Concatenates the text representations of all the arguments.
        NULL arguments are ignored.
       </para>
       <para>
        <literal>concat('abcde', 2, NULL, 22)</literal>
        <returnvalue>abcde222</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>concat_ws</primary>
        </indexterm>
        <function>concat_ws</function> ( <parameter>sep</parameter> <type>text</type>,
        <parameter>val1</parameter> <type>"any"</type>
        <optional>, <parameter>val2</parameter> <type>"any"</type> <optional>, ...</optional> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Concatenates all but the first argument, with separators. The first
        argument is used as the separator string, and should not be NULL.
        Other NULL arguments are ignored.
       </para>
       <para>
        <literal>concat_ws(',', 'abcde', 2, NULL, 22)</literal>
        <returnvalue>abcde,2,22</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>format</primary>
        </indexterm>
        <function>format</function> ( <parameter>formatstr</parameter> <type>text</type>
        <optional>, <parameter>formatarg</parameter> <type>"any"</type> <optional>, ...</optional> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
         Formats arguments according to a format string;
         see <xref linkend="functions-string-format"/>.
         This function is similar to the C function <function>sprintf</function>.
       </para>
       <para>
        <literal>format('Hello %s, %1$s', 'World')</literal>
        <returnvalue>Hello World, World</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>initcap</primary>
        </indexterm>
        <function>initcap</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the first letter of each word to upper case (or title case
        if the letter is a digraph and locale is <literal>ICU</literal> or
        <literal>builtin</literal> <literal>PG_UNICODE_FAST</literal>)
        and the rest to lower case. When using the <literal>libc</literal> or
        <literal>builtin</literal> locale provider, words are sequences of
        alphanumeric characters separated by non-alphanumeric characters;
        when using the ICU locale provider, words are separated according to
        <ulink url="https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/ustring_8h.html#a47602e2c2012d77ee91908b9bbfdc063">u_strToTitle ICU function</ulink>.
       </para>
       <para>
        This function is primarily used for convenient
        display, and the specific result should not be relied upon because of
        the differences between locale providers and between different
        ICU versions. If specific word boundary rules are desired,
        it is recommended to write a custom function.
       </para>
       <para>
        <literal>initcap('hi THOMAS')</literal>
        <returnvalue>Hi Thomas</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>casefold</primary>
        </indexterm>
        <function>casefold</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Performs case folding of the input string according to the collation.
        Case folding is similar to case conversion, but the purpose of case
        folding is to facilitate case-insensitive matching of strings,
        whereas the purpose of case conversion is to convert to a particular
        cased form.  This function can only be used when the server encoding
        is <literal>UTF8</literal>.
       </para>
       <para>
        Ordinarily, case folding simply converts to lowercase, but there may
        be exceptions depending on the collation.  For instance, some
        characters have more than two lowercase variants, or fold to uppercase.
       </para>
       <para>
        Case folding may change the length of the string.  For instance, in
        the <literal>PG_UNICODE_FAST</literal> collation, <literal>ß</literal>
        (U+00DF) folds to <literal>ss</literal>.
       </para>
       <para>
        <function>casefold</function> can be used for Unicode Default Caseless
        Matching.  It does not always preserve the normalized form of the
        input string (see <xref linkend="function-normalize"/>).
       </para>
       <para>
        The <literal>libc</literal> provider doesn't support case folding, so
        <function>casefold</function> is identical to <xref
        linkend="function-lower"/>.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>left</primary>
        </indexterm>
        <function>left</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>n</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns first <parameter>n</parameter> characters in the
        string, or when <parameter>n</parameter> is negative, returns
        all but last |<parameter>n</parameter>| characters.
       </para>
       <para>
        <literal>left('abcde', 2)</literal>
        <returnvalue>ab</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>length</primary>
        </indexterm>
        <function>length</function> ( <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of characters in the string.
       </para>
       <para>
        <literal>length('jose')</literal>
        <returnvalue>4</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>md5</primary>
        </indexterm>
        <function>md5</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Computes the MD5 <link linkend="functions-hash-note">hash</link> of
        the argument, with the result written in hexadecimal.
       </para>
       <para>
        <literal>md5('abc')</literal>
        <returnvalue>900150983cd24fb0&zwsp;d6963f7d28e17f72</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>parse_ident</primary>
        </indexterm>
        <function>parse_ident</function> ( <parameter>qualified_identifier</parameter> <type>text</type>
        <optional>, <parameter>strict_mode</parameter> <type>boolean</type> <literal>DEFAULT</literal> <literal>true</literal> </optional> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Splits <parameter>qualified_identifier</parameter> into an array of
        identifiers, removing any quoting of individual identifiers.  By
        default, extra characters after the last identifier are considered an
        error; but if the second parameter is <literal>false</literal>, then such
        extra characters are ignored. (This behavior is useful for parsing
        names for objects like functions.) Note that this function does not
        truncate over-length identifiers. If you want truncation you can cast
        the result to <type>name[]</type>.
       </para>
       <para>
        <literal>parse_ident('"SomeSchema".someTable')</literal>
        <returnvalue>{SomeSchema,sometable}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>pg_client_encoding</primary>
        </indexterm>
        <function>pg_client_encoding</function> ( )
        <returnvalue>name</returnvalue>
       </para>
       <para>
        Returns current client encoding name.
       </para>
       <para>
        <literal>pg_client_encoding()</literal>
        <returnvalue>UTF8</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>quote_ident</primary>
        </indexterm>
        <function>quote_ident</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the given string suitably quoted to be used as an identifier
        in an <acronym>SQL</acronym> statement string.
        Quotes are added only if necessary (i.e., if the string contains
        non-identifier characters or would be case-folded).
        Embedded quotes are properly doubled.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </para>
       <para>
        <literal>quote_ident('Foo bar')</literal>
        <returnvalue>"Foo bar"</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>quote_literal</primary>
        </indexterm>
        <function>quote_literal</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the given string suitably quoted to be used as a string literal
        in an <acronym>SQL</acronym> statement string.
        Embedded single-quotes and backslashes are properly doubled.
        Note that <function>quote_literal</function> returns null on null
        input; if the argument might be null,
        <function>quote_nullable</function> is often more suitable.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </para>
       <para>
        <literal>quote_literal(E'O\'Reilly')</literal>
        <returnvalue>'O''Reilly'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>quote_literal</function> ( <type>anyelement</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the given value to text and then quotes it as a literal.
        Embedded single-quotes and backslashes are properly doubled.
       </para>
       <para>
        <literal>quote_literal(42.5)</literal>
        <returnvalue>'42.5'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>quote_nullable</primary>
        </indexterm>
        <function>quote_nullable</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the given string suitably quoted to be used as a string literal
        in an <acronym>SQL</acronym> statement string; or, if the argument
        is null, returns <literal>NULL</literal>.
        Embedded single-quotes and backslashes are properly doubled.
        See also <xref linkend="plpgsql-quote-literal-example"/>.
       </para>
       <para>
        <literal>quote_nullable(NULL)</literal>
        <returnvalue>NULL</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>quote_nullable</function> ( <type>anyelement</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the given value to text and then quotes it as a literal;
        or, if the argument is null, returns <literal>NULL</literal>.
        Embedded single-quotes and backslashes are properly doubled.
       </para>
       <para>
        <literal>quote_nullable(42.5)</literal>
        <returnvalue>'42.5'</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_count</primary>
        </indexterm>
        <function>regexp_count</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>
         <optional>, <parameter>start</parameter> <type>integer</type>
         <optional>, <parameter>flags</parameter> <type>text</type> </optional> </optional> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the number of times the POSIX regular
        expression <parameter>pattern</parameter> matches in
        the <parameter>string</parameter>; see
        <xref linkend="functions-posix-regexp-count"/>.
       </para>
       <para>
        <literal>regexp_count('123456789012', '\d\d\d', 2)</literal>
        <returnvalue>3</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_instr</primary>
        </indexterm>
        <function>regexp_instr</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>
         <optional>, <parameter>start</parameter> <type>integer</type>
         <optional>, <parameter>N</parameter> <type>integer</type>
         <optional>, <parameter>endoption</parameter> <type>integer</type>
         <optional>, <parameter>flags</parameter> <type>text</type>
         <optional>, <parameter>subexpr</parameter> <type>integer</type> </optional> </optional> </optional> </optional> </optional> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns the position within <parameter>string</parameter> where
        the <parameter>N</parameter>'th match of the POSIX regular
        expression <parameter>pattern</parameter> occurs, or zero if there is
        no such match; see <xref linkend="functions-posix-regexp-instr"/>.
       </para>
       <para>
        <literal>regexp_instr('ABCDEF', 'c(.)(..)', 1, 1, 0, 'i')</literal>
        <returnvalue>3</returnvalue>
       </para>
       <para>
        <literal>regexp_instr('ABCDEF', 'c(.)(..)', 1, 1, 0, 'i', 2)</literal>
        <returnvalue>5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_like</primary>
        </indexterm>
        <function>regexp_like</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>
         <optional>, <parameter>flags</parameter> <type>text</type> </optional> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Checks whether a match of the POSIX regular
        expression <parameter>pattern</parameter> occurs
        within <parameter>string</parameter>; see
        <xref linkend="functions-posix-regexp-like"/>.
       </para>
       <para>
        <literal>regexp_like('Hello World', 'world$', 'i')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_match</primary>
        </indexterm>
        <function>regexp_match</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> <optional>, <parameter>flags</parameter> <type>text</type> </optional> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Returns substrings within the first match of the POSIX regular
        expression <parameter>pattern</parameter> to
        the <parameter>string</parameter>; see
        <xref linkend="functions-posix-regexp-match"/>.
       </para>
       <para>
        <literal>regexp_match('foobarbequebaz', '(bar)(beque)')</literal>
        <returnvalue>{bar,beque}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_matches</primary>
        </indexterm>
        <function>regexp_matches</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> <optional>, <parameter>flags</parameter> <type>text</type> </optional> )
        <returnvalue>setof text[]</returnvalue>
       </para>
       <para>
        Returns substrings within the first match of the POSIX regular
        expression <parameter>pattern</parameter> to
        the <parameter>string</parameter>, or substrings within all
        such matches if the <literal>g</literal> flag is used;
        see <xref linkend="functions-posix-regexp-matches"/>.
       </para>
       <para>
        <literal>regexp_matches('foobarbequebaz', 'ba.', 'g')</literal>
        <returnvalue></returnvalue>
<programlisting>
 {bar}
 {baz}
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_replace</primary>
        </indexterm>
        <function>regexp_replace</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>, <parameter>replacement</parameter> <type>text</type>
         <optional>, <parameter>flags</parameter> <type>text</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces the substring that is the first match to the POSIX
        regular expression <parameter>pattern</parameter>, or all such
        matches if the <literal>g</literal> flag is used; see
        <xref linkend="functions-posix-regexp-replace"/>.
       </para>
       <para>
        <literal>regexp_replace('Thomas', '.[mN]a.', 'M')</literal>
        <returnvalue>ThM</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>regexp_replace</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>, <parameter>replacement</parameter> <type>text</type>,
         <parameter>start</parameter> <type>integer</type>
         <optional>, <parameter>N</parameter> <type>integer</type>
         <optional>, <parameter>flags</parameter> <type>text</type> </optional> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces the substring that is the <parameter>N</parameter>'th
        match to the POSIX regular expression <parameter>pattern</parameter>,
        or all such matches if <parameter>N</parameter> is zero, with the
        search beginning at the <parameter>start</parameter>'th character
        of <parameter>string</parameter>.  If <parameter>N</parameter> is
        omitted, it defaults to 1.  See
        <xref linkend="functions-posix-regexp-replace"/>.
       </para>
       <para>
        <literal>regexp_replace('Thomas', '.', 'X', 3, 2)</literal>
        <returnvalue>ThoXas</returnvalue>
       </para>
       <para>
        <literal>regexp_replace(string=>'hello world', pattern=>'l', replacement=>'XX', start=>1, "N"=>2)</literal>
        <returnvalue>helXXo world</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_split_to_array</primary>
        </indexterm>
        <function>regexp_split_to_array</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> <optional>, <parameter>flags</parameter> <type>text</type> </optional> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Splits <parameter>string</parameter> using a POSIX regular
        expression as the delimiter, producing an array of results; see
        <xref linkend="functions-posix-regexp-split-to-array"/>.
       </para>
       <para>
        <literal>regexp_split_to_array('hello world', '\s+')</literal>
        <returnvalue>{hello,world}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_split_to_table</primary>
        </indexterm>
        <function>regexp_split_to_table</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type> <optional>, <parameter>flags</parameter> <type>text</type> </optional> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Splits <parameter>string</parameter> using a POSIX regular
        expression as the delimiter, producing a set of results; see
        <xref linkend="functions-posix-regexp-split-to-table"/>.
       </para>
       <para>
        <literal>regexp_split_to_table('hello world', '\s+')</literal>
        <returnvalue></returnvalue>
<programlisting>
 hello
 world
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>regexp_substr</primary>
        </indexterm>
        <function>regexp_substr</function> ( <parameter>string</parameter> <type>text</type>, <parameter>pattern</parameter> <type>text</type>
         <optional>, <parameter>start</parameter> <type>integer</type>
         <optional>, <parameter>N</parameter> <type>integer</type>
         <optional>, <parameter>flags</parameter> <type>text</type>
         <optional>, <parameter>subexpr</parameter> <type>integer</type> </optional> </optional> </optional> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns the substring within <parameter>string</parameter> that
        matches the <parameter>N</parameter>'th occurrence of the POSIX
        regular expression <parameter>pattern</parameter>,
        or <literal>NULL</literal> if there is no such match; see
        <xref linkend="functions-posix-regexp-substr"/>.
       </para>
       <para>
        <literal>regexp_substr('ABCDEF', 'c(.)(..)', 1, 1, 'i')</literal>
        <returnvalue>CDEF</returnvalue>
       </para>
       <para>
        <literal>regexp_substr('ABCDEF', 'c(.)(..)', 1, 1, 'i', 2)</literal>
        <returnvalue>EF</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>repeat</primary>
        </indexterm>
        <function>repeat</function> ( <parameter>string</parameter> <type>text</type>, <parameter>number</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Repeats <parameter>string</parameter> the specified
        <parameter>number</parameter> of times.
       </para>
       <para>
        <literal>repeat('Pg', 4)</literal>
        <returnvalue>PgPgPgPg</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>replace</primary>
        </indexterm>
        <function>replace</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces all occurrences in <parameter>string</parameter> of
        substring <parameter>from</parameter> with
        substring <parameter>to</parameter>.
       </para>
       <para>
        <literal>replace('abcdefabcdef', 'cd', 'XX')</literal>
        <returnvalue>abXXefabXXef</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>reverse</primary>
        </indexterm>
        <function>reverse</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Reverses the order of the characters in the string.
       </para>
       <para>
        <literal>reverse('abcde')</literal>
        <returnvalue>edcba</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>right</primary>
        </indexterm>
        <function>right</function> ( <parameter>string</parameter> <type>text</type>,
         <parameter>n</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Returns last <parameter>n</parameter> characters in the string,
        or when <parameter>n</parameter> is negative, returns all but
        first |<parameter>n</parameter>| characters.
       </para>
       <para>
        <literal>right('abcde', 2)</literal>
        <returnvalue>de</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>split_part</primary>
        </indexterm>
        <function>split_part</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>delimiter</parameter> <type>text</type>,
        <parameter>n</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Splits <parameter>string</parameter> at occurrences
        of <parameter>delimiter</parameter> and returns
        the <parameter>n</parameter>'th field (counting from one),
        or when <parameter>n</parameter> is negative, returns
        the |<parameter>n</parameter>|'th-from-last field.
       </para>
       <para>
        <literal>split_part('abc~@~def~@~ghi', '~@~', 2)</literal>
        <returnvalue>def</returnvalue>
       </para>
       <para>
        <literal>split_part('abc,def,ghi,jkl', ',', -2)</literal>
        <returnvalue>ghi</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>starts_with</primary>
        </indexterm>
        <function>starts_with</function> ( <parameter>string</parameter> <type>text</type>, <parameter>prefix</parameter> <type>text</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns true if <parameter>string</parameter> starts
        with <parameter>prefix</parameter>.
       </para>
       <para>
        <literal>starts_with('alphabet', 'alph')</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm id="function-string-to-array">
         <primary>string_to_array</primary>
        </indexterm>
        <function>string_to_array</function> ( <parameter>string</parameter> <type>text</type>, <parameter>delimiter</parameter> <type>text</type> <optional>, <parameter>null_string</parameter> <type>text</type> </optional> )
        <returnvalue>text[]</returnvalue>
       </para>
       <para>
        Splits the <parameter>string</parameter> at occurrences
        of <parameter>delimiter</parameter> and forms the resulting fields
        into a <type>text</type> array.
        If <parameter>delimiter</parameter> is <literal>NULL</literal>,
        each character in the <parameter>string</parameter> will become a
        separate element in the array.
        If <parameter>delimiter</parameter> is an empty string, then
        the <parameter>string</parameter> is treated as a single field.
        If <parameter>null_string</parameter> is supplied and is
        not <literal>NULL</literal>, fields matching that string are
        replaced by <literal>NULL</literal>.
        See also <link linkend="function-array-to-string"><function>array_to_string</function></link>.
       </para>
       <para>
        <literal>string_to_array('xx~~yy~~zz', '~~', 'yy')</literal>
        <returnvalue>{xx,NULL,zz}</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>string_to_table</primary>
        </indexterm>
        <function>string_to_table</function> ( <parameter>string</parameter> <type>text</type>, <parameter>delimiter</parameter> <type>text</type> <optional>, <parameter>null_string</parameter> <type>text</type> </optional> )
        <returnvalue>setof text</returnvalue>
       </para>
       <para>
        Splits the <parameter>string</parameter> at occurrences
        of <parameter>delimiter</parameter> and returns the resulting fields
        as a set of <type>text</type> rows.
        If <parameter>delimiter</parameter> is <literal>NULL</literal>,
        each character in the <parameter>string</parameter> will become a
        separate row of the result.
        If <parameter>delimiter</parameter> is an empty string, then
        the <parameter>string</parameter> is treated as a single field.
        If <parameter>null_string</parameter> is supplied and is
        not <literal>NULL</literal>, fields matching that string are
        replaced by <literal>NULL</literal>.
       </para>
       <para>
        <literal>string_to_table('xx~^~yy~^~zz', '~^~', 'yy')</literal>
        <returnvalue></returnvalue>
<programlisting>
 xx
 NULL
 zz
</programlisting>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>strpos</primary>
        </indexterm>
        <function>strpos</function> ( <parameter>string</parameter> <type>text</type>, <parameter>substring</parameter> <type>text</type> )
        <returnvalue>integer</returnvalue>
       </para>
       <para>
        Returns first starting index of the specified <parameter>substring</parameter>
        within <parameter>string</parameter>, or zero if it's not present.
        (Same as <literal>position(<parameter>substring</parameter> in
        <parameter>string</parameter>)</literal>, but note the reversed
        argument order.)
       </para>
       <para>
        <literal>strpos('high', 'ig')</literal>
        <returnvalue>2</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>substr</primary>
        </indexterm>
        <function>substr</function> ( <parameter>string</parameter> <type>text</type>, <parameter>start</parameter> <type>integer</type> <optional>, <parameter>count</parameter> <type>integer</type> </optional> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Extracts the substring of <parameter>string</parameter> starting at
        the <parameter>start</parameter>'th character,
        and extending for <parameter>count</parameter> characters if that is
        specified.  (Same
        as <literal>substring(<parameter>string</parameter>
        from <parameter>start</parameter>
        for <parameter>count</parameter>)</literal>.)
       </para>
       <para>
        <literal>substr('alphabet', 3)</literal>
        <returnvalue>phabet</returnvalue>
       </para>
       <para>
        <literal>substr('alphabet', 3, 2)</literal>
        <returnvalue>ph</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_ascii</primary>
        </indexterm>
        <function>to_ascii</function> ( <parameter>string</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_ascii</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>encoding</parameter> <type>name</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_ascii</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>encoding</parameter> <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts <parameter>string</parameter> to <acronym>ASCII</acronym>
        from another encoding, which may be identified by name or number.
        If <parameter>encoding</parameter> is omitted the database encoding
        is assumed (which in practice is the only useful case).
        The conversion consists primarily of dropping accents.
        Conversion is only supported
        from <literal>LATIN1</literal>, <literal>LATIN2</literal>,
        <literal>LATIN9</literal>, and <literal>WIN1250</literal> encodings.
        (See the <xref linkend="unaccent"/> module for another, more flexible
        solution.)
       </para>
       <para>
        <literal>to_ascii('Kar&eacute;l')</literal>
        <returnvalue>Karel</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_bin</primary>
        </indexterm>
        <function>to_bin</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_bin</function> ( <type>bigint</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the number to its equivalent two's complement binary
        representation.
       </para>
       <para>
        <literal>to_bin(2147483647)</literal>
        <returnvalue>1111111111111111111111111111111</returnvalue>
       </para>
       <para>
        <literal>to_bin(-1234)</literal>
        <returnvalue>11111111111111111111101100101110</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_hex</primary>
        </indexterm>
        <function>to_hex</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_hex</function> ( <type>bigint</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the number to its equivalent two's complement hexadecimal
        representation.
       </para>
       <para>
        <literal>to_hex(2147483647)</literal>
        <returnvalue>7fffffff</returnvalue>
       </para>
       <para>
        <literal>to_hex(-1234)</literal>
        <returnvalue>fffffb2e</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>to_oct</primary>
        </indexterm>
        <function>to_oct</function> ( <type>integer</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para role="func_signature">
        <function>to_oct</function> ( <type>bigint</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Converts the number to its equivalent two's complement octal
        representation.
       </para>
       <para>
        <literal>to_oct(2147483647)</literal>
        <returnvalue>17777777777</returnvalue>
       </para>
       <para>
        <literal>to_oct(-1234)</literal>
        <returnvalue>37777775456</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>translate</primary>
        </indexterm>
        <function>translate</function> ( <parameter>string</parameter> <type>text</type>,
        <parameter>from</parameter> <type>text</type>,
        <parameter>to</parameter> <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Replaces each character in <parameter>string</parameter> that
        matches a character in the <parameter>from</parameter> set with the
        corresponding character in the <parameter>to</parameter>
        set. If <parameter>from</parameter> is longer than
        <parameter>to</parameter>, occurrences of the extra characters in
        <parameter>from</parameter> are deleted.
       </para>
       <para>
        <literal>translate('12345', '143', 'ax')</literal>
        <returnvalue>a2x5</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm>
         <primary>unistr</primary>
        </indexterm>
        <function>unistr</function> ( <type>text</type> )
        <returnvalue>text</returnvalue>
       </para>
       <para>
        Evaluate escaped Unicode characters in the argument.  Unicode characters
        can be specified as
        <literal>\<replaceable>XXXX</replaceable></literal> (4 hexadecimal
        digits), <literal>\+<replaceable>XXXXXX</replaceable></literal> (6
        hexadecimal digits),
        <literal>\u<replaceable>XXXX</replaceable></literal> (4 hexadecimal
        digits), or <literal>\U<replaceable>XXXXXXXX</replaceable></literal>
        (8 hexadecimal digits).  To specify a backslash, write two
        backslashes.  All other characters are taken literally.
       </para>

       <para>
        If the server encoding is not UTF-8, the Unicode code point identified
        by one of these escape sequences is converted to the actual server
        encoding; an error is reported if that's not possible.
       </para>

       <para>
        This function provides a (non-standard) alternative to string
        constants with Unicode escapes (see <xref
        linkend="sql-syntax-strings-uescape"/>).
       </para>

       <para>
        <literal>unistr('d\0061t\+000061')</literal>
        <returnvalue>data</returnvalue>
       </para>
       <para>
        <literal>unistr('d\u0061t\U00000061')</literal>
        <returnvalue>data</returnvalue>
       </para></entry>
      </row>

     </tbody>
    </tgroup>
   </table>

   <para>
    The <function>concat</function>, <function>concat_ws</function> and
    <function>format</function> functions are variadic, so it is possible to
    pass the values to be concatenated or formatted as an array marked with
    the <literal>VARIADIC</literal> keyword (see <xref
    linkend="xfunc-sql-variadic-functions"/>).  The array's elements are
    treated as if they were separate ordinary arguments to the function.
    If the variadic array argument is NULL, <function>concat</function>
    and <function>concat_ws</function> return NULL, but
    <function>format</function> treats a NULL as a zero-element array.
   </para>

   <para>
    See also the aggregate function <function>string_agg</function> in
    <xref linkend="functions-aggregate"/>, and the functions for
    converting between strings and the <type>bytea</type> type in
    <xref linkend="functions-binarystring-conversions"/>.
   </para>

   <sect2 id="functions-string-format">
    <title><function>format</function></title>

    <indexterm>
     <primary>format</primary>
    </indexterm>

    <para>
     The function <function>format</function> produces output formatted according to
     a format string, in a style similar to the C function
     <function>sprintf</function>.
    </para>

    <para>
<synopsis>
<function>format</function>(<parameter>formatstr</parameter> <type>text</type> <optional>, <parameter>formatarg</parameter> <type>"any"</type> <optional>, ...</optional> </optional>)
</synopsis>
     <parameter>formatstr</parameter> is a format string that specifies how the
     result should be formatted.  Text in the format string is copied
     directly to the result, except where <firstterm>format specifiers</firstterm> are
     used.  Format specifiers act as placeholders in the string, defining how
     subsequent function arguments should be formatted and inserted into the
     result.  Each <parameter>formatarg</parameter> argument is converted to text
     according to the usual output rules for its data type, and then formatted
     and inserted into the result string according to the format specifier(s).
    </para>

    <para>
     Format specifiers are introduced by a <literal>%</literal> character and have
     the form
<synopsis>
%[<parameter>position</parameter>][<parameter>flags</parameter>][<parameter>width</parameter>]<parameter>type</parameter>
</synopsis>
     where the component fields are:

     <variablelist>
      <varlistentry>
       <term><parameter>position</parameter> (optional)</term>
       <listitem>
        <para>
         A string of the form <literal><parameter>n</parameter>$</literal> where
         <parameter>n</parameter> is the index of the argument to print.
         Index 1 means the first argument after
         <parameter>formatstr</parameter>.  If the <parameter>position</parameter> is
         omitted, the default is to use the next argument in sequence.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><parameter>flags</parameter> (optional)</term>
       <listitem>
        <para>
         Additional options controlling how the format specifier's output is
         formatted.  Currently the only supported flag is a minus sign
         (<literal>-</literal>) which will cause the format specifier's output to be
         left-justified.  This has no effect unless the <parameter>width</parameter>
         field is also specified.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><parameter>width</parameter> (optional)</term>
       <listitem>
        <para>
         Specifies the <emphasis>minimum</emphasis> number of characters to use to
         display the format specifier's output.  The output is padded on the
         left or right (depending on the <literal>-</literal> flag) with spaces as
         needed to fill the width.  A too-small width does not cause
         truncation of the output, but is simply ignored.  The width may be
         specified using any of the following: a positive integer; an
         asterisk (<literal>*</literal>) to use the next function argument as the
         width; or a string of the form <literal>*<parameter>n</parameter>$</literal> to
         use the <parameter>n</parameter>th function argument as the width.
        </para>

        <para>
         If the width comes from a function argument, that argument is
         consumed before the argument that is used for the format specifier's
         value.  If the width argument is negative, the result is left
         aligned (as if the <literal>-</literal> flag had been specified) within a
         field of length <function>abs</function>(<parameter>width</parameter>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><parameter>type</parameter> (required)</term>
       <listitem>
        <para>
         The type of format conversion to use to produce the format
         specifier's output.  The following types are supported:
         <itemizedlist>
          <listitem>
           <para>
            <literal>s</literal> formats the argument value as a simple
            string.  A null value is treated as an empty string.
           </para>
          </listitem>
          <listitem>
           <para>
            <literal>I</literal> treats the argument value as an SQL
            identifier, double-quoting it if necessary.
            It is an error for the value to be null (equivalent to
            <function>quote_ident</function>).
           </para>
          </listitem>
          <listitem>
           <para>
            <literal>L</literal> quotes the argument value as an SQL literal.
            A null value is displayed as the string <literal>NULL</literal>, without
            quotes (equivalent to <function>quote_nullable</function>).
           </para>
          </listitem>
         </itemizedlist>
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

    <para>
     In addition to the format specifiers described above, the special sequence
     <literal>%%</literal> may be used to output a literal <literal>%</literal> character.
    </para>

    <para>
     Here are some examples of the basic format conversions:

<screen>
SELECT format('Hello %s', 'World');
<lineannotation>Result: </lineannotation><computeroutput>Hello World</computeroutput>

SELECT format('Testing %s, %s, %s, %%', 'one', 'two', 'three');
<lineannotation>Result: </lineannotation><computeroutput>Testing one, two, three, %</computeroutput>

SELECT format('INSERT INTO %I VALUES(%L)', 'Foo bar', E'O\'Reilly');
<lineannotation>Result: </lineannotation><computeroutput>INSERT INTO "Foo bar" VALUES('O''Reilly')</computeroutput>

SELECT format('INSERT INTO %I VALUES(%L)', 'locations', 'C:\Program Files');
<lineannotation>Result: </lineannotation><computeroutput>INSERT INTO locations VALUES('C:\Program Files')</computeroutput>
</screen>
    </para>

    <para>
     Here are examples using <parameter>width</parameter> fields
     and the <literal>-</literal> flag:

<screen>
SELECT format('|%10s|', 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|       foo|</computeroutput>

SELECT format('|%-10s|', 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>

SELECT format('|%*s|', 10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|       foo|</computeroutput>

SELECT format('|%*s|', -10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>

SELECT format('|%-*s|', 10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>

SELECT format('|%-*s|', -10, 'foo');
<lineannotation>Result: </lineannotation><computeroutput>|foo       |</computeroutput>
</screen>
    </para>

    <para>
     These examples show use of <parameter>position</parameter> fields:

<screen>
SELECT format('Testing %3$s, %2$s, %1$s', 'one', 'two', 'three');
<lineannotation>Result: </lineannotation><computeroutput>Testing three, two, one</computeroutput>

SELECT format('|%*2$s|', 'foo', 10, 'bar');
<lineannotation>Result: </lineannotation><computeroutput>|       bar|</computeroutput>

SELECT format('|%1$*2$s|', 'foo', 10, 'bar');
<lineannotation>Result: </lineannotation><computeroutput>|       foo|</computeroutput>
</screen>
    </para>

    <para>
     Unlike the standard C function <function>sprintf</function>,
     <productname>PostgreSQL</productname>'s <function>format</function> function allows format
     specifiers with and without <parameter>position</parameter> fields to be mixed
     in the same format string.  A format specifier without a
     <parameter>position</parameter> field always uses the next argument after the
     last argument consumed.
     In addition, the <function>format</function> function does not require all
     function arguments to be used in the format string.
     For example:

<screen>
SELECT format('Testing %3$s, %2$s, %s', 'one', 'two', 'three');
<lineannotation>Result: </lineannotation><computeroutput>Testing three, two, three</computeroutput>
</screen>
    </para>

    <para>
     The <literal>%I</literal> and <literal>%L</literal> format specifiers are particularly
     useful for safely constructing dynamic SQL statements.  See
     <xref linkend="plpgsql-quote-literal-example"/>.
    </para>
   </sect2>

  </sect1>