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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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91
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 --> <!-- end re_syntax.n man page -->
<sect3 id="posix-vs-xquery"> <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"> <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>
<indexterm zone="posix-vs-xquery"> <indexterm zone="posix-vs-xquery">
@ -7364,16 +7380,75 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
</indexterm> </indexterm>
<para> <para>
Since SQL:2008, the SQL standard includes Since SQL:2008, the SQL standard includes regular expression operators
a <literal>LIKE_REGEX</literal> operator that performs pattern and functions that performs pattern
matching according to the XQuery regular expression matching according to the XQuery regular expression
standard. <productname>PostgreSQL</productname> does not yet standard:
implement this operator, but you can get very similar behavior using <itemizedlist>
the <function>regexp_match()</function> function, since XQuery <listitem><para><literal>LIKE_REGEX</literal></para></listitem>
regular expressions are quite close to the ARE syntax described above. <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>
<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 Notable differences between the existing POSIX-based
regular-expression feature and XQuery regular expressions include: regular-expression feature and XQuery regular expressions include:

10
src/backend/catalog/sql_features.txt
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@ -323,11 +323,11 @@ F821 Local table references NO
F831 Full cursor update NO F831 Full cursor update NO
F831 Full cursor update 01 Updatable scrollable cursors NO F831 Full cursor update 01 Updatable scrollable cursors NO
F831 Full cursor update 02 Updatable ordered cursors NO F831 Full cursor update 02 Updatable ordered cursors NO
F841 LIKE_REGEX predicate NO F841 LIKE_REGEX predicate NO consider regexp_like()
F842 OCCURRENCES_REGEX function NO F842 OCCURRENCES_REGEX function NO consider regexp_matches()
F843 POSITION_REGEX function NO F843 POSITION_REGEX function NO consider regexp_instr()
F844 SUBSTRING_REGEX function NO F844 SUBSTRING_REGEX function NO consider regexp_substr()
F845 TRANSLATE_REGEX function NO F845 TRANSLATE_REGEX function NO consider regexp_replace()
F846 Octet support in regular expression operators NO F846 Octet support in regular expression operators NO
F847 Nonconstant regular expressions NO F847 Nonconstant regular expressions NO
F850 Top-level <order by clause> in <query expression> YES F850 Top-level <order by clause> in <query expression> YES