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doc: More documentation on regular expressions and SQL standard

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Reviewed-by: Gilles Darold <gilles@darold.net>
Discussion: https://www.postgresql.org/message-id/b7988566-daa2-80ed-2fdc-6f6630462d26@enterprisedb.com
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Peter Eisentraut
2021-12-20 10:12:25 +01:00
parent b1c169caf0
commit 222b697ec0
2 changed files with 88 additions and 13 deletions
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doc/src/sgml/func.sgml
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@@ -7353,10 +7353,26 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
<!-- end re_syntax.n man page -->
<sect3 id="posix-vs-xquery">
<title>Differences from XQuery (<literal>LIKE_REGEX</literal>)</title>
<title>Differences from SQL Standard and XQuery</title>
<indexterm zone="posix-vs-xquery">
<primary><literal>LIKE_REGEX</literal></primary>
<primary>LIKE_REGEX</primary>
</indexterm>
<indexterm zone="posix-vs-xquery">
<primary>OCCURRENCES_REGEX</primary>
</indexterm>
<indexterm zone="posix-vs-xquery">
<primary>POSITION_REGEX</primary>
</indexterm>
<indexterm zone="posix-vs-xquery">
<primary>SUBSTRING_REGEX</primary>
</indexterm>
<indexterm zone="posix-vs-xquery">
<primary>TRANSLATE_REGEX</primary>
</indexterm>
<indexterm zone="posix-vs-xquery">
@@ -7364,16 +7380,75 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
</indexterm>
<para>
Since SQL:2008, the SQL standard includes
a <literal>LIKE_REGEX</literal> operator that performs pattern
Since SQL:2008, the SQL standard includes regular expression operators
and functions that performs pattern
matching according to the XQuery regular expression
standard. <productname>PostgreSQL</productname> does not yet
implement this operator, but you can get very similar behavior using
the <function>regexp_match()</function> function, since XQuery
regular expressions are quite close to the ARE syntax described above.
standard:
<itemizedlist>
<listitem><para><literal>LIKE_REGEX</literal></para></listitem>
<listitem><para><literal>OCCURRENCES_REGEX</literal></para></listitem>
<listitem><para><literal>POSITION_REGEX</literal></para></listitem>
<listitem><para><literal>SUBSTRING_REGEX</literal></para></listitem>
<listitem><para><literal>TRANSLATE_REGEX</literal></para></listitem>
</itemizedlist>
<productname>PostgreSQL</productname> does not currently implement these
operators and functions. You can get approximately equivalent
functionality in each case as shown in <xref
linkend="functions-regexp-sql-table"/>. (Various optional clauses on
both sides have been omitted in this table.)
</para>
<table id="functions-regexp-sql-table">
<title>Regular Expression Functions Equivalencies</title>
<tgroup cols="2">
<thead>
<row>
<entry>SQL standard</entry>
<entry>PostgreSQL</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal><replaceable>string</replaceable> LIKE_REGEX <replaceable>pattern</replaceable></literal></entry>
<entry><literal>regexp_like(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal> or <literal><replaceable>string</replaceable> ~ <replaceable>pattern</replaceable></literal></entry>
</row>
<row>
<entry><literal>OCCURRENCES_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable></literal></entry>
<entry><literal>regexp_count(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal></entry>
</row>
<row>
<entry><literal>POSITION_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable></literal></entry>
<entry><literal>regexp_instr(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal></entry>
</row>
<row>
<entry><literal>SUBSTRING_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable></literal></entry>
<entry><literal>regexp_substr(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>)</literal></entry>
</row>
<row>
<entry><literal>TRANSLATE_REGEX(<replaceable>pattern</replaceable> IN <replaceable>string</replaceable> WITH <replaceable>replacement</replaceable></literal></entry>
<entry><literal>regexp_replace(<replaceable>string</replaceable>, <replaceable>pattern</replaceable>, <replaceable>replacement</replaceable>)</literal></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Regular expression functions similar to those provided by PostgreSQL are
also available in a number of other SQL implementations, whereas the
SQL-standard functions are not as widely implemented. Some of the
details of the regular expression syntax will likely differ in each
implementation.
</para>
<para>
The SQL-standard operators and functions use XQuery regular expressions,
which are quite close to the ARE syntax described above.
Notable differences between the existing POSIX-based
regular-expression feature and XQuery regular expressions include: