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Commit0a97edb

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Doc: improve documentation around jsonpath regular expressions.
Provide some documentation about the differences between XQueryregular expressions and those supported by Spencer's regex engine.Since SQL now exposes XQuery regexps with the LIKE_REGEX operator,I made this a standalone section designed to help somebody whohas to translate a LIKE_REGEX query to Postgres. (Eventually we mightextend Spencer's engine to allow precise implementation of XQuery,but not today.)Reference that in the jsonpath docs, provide definitions of theXQuery flag letters, and add a description of the JavaScript-inspiredstring literal syntax used within jsonpath. Also point out explicitlythat backslashes used within like_regex patterns will need to be doubled.This also syncs the docs with the decision implemented in commitd5b90cd to desupport XQuery's 'x' flag for now.Jonathan Katz and Tom LaneDiscussion:https://postgr.es/m/CAPpHfdvDci4iqNF9fhRkTqhe-5_8HmzeLt56drH%2B_Rv2rNRqfg@mail.gmail.com
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‎doc/src/sgml/func.sgml‎

Lines changed: 209 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -5970,6 +5970,145 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
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<!-- end re_syntax.n man page -->
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<sect3 id="posix-vs-xquery">
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<title>Differences From XQuery (<literal>LIKE_REGEX</literal>)</title>
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5976+
<indexterm zone="posix-vs-xquery">
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<primary><literal>LIKE_REGEX</literal></primary>
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</indexterm>
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<indexterm zone="posix-vs-xquery">
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<primary>XQuery regular expressions</primary>
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</indexterm>
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<para>
5985+
Since SQL:2008, the SQL standard includes
5986+
a <literal>LIKE_REGEX</literal> operator that performs pattern
5987+
matching according to the XQuery regular expression
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standard. <productname>PostgreSQL</productname> does not yet
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implement this operator, but you can get very similar behavior using
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the <function>regexp_match()</function> function, since XQuery
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regular expressions are quite close to the ARE syntax described above.
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</para>
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<para>
5995+
Notable differences between the existing POSIX-based
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regular-expression feature and XQuery regular expressions include:
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<itemizedlist>
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<listitem>
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<para>
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XQuery character class subtraction is not supported. An example of
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this feature is using the following to match only English
6003+
consonants: <literal>[a-z-[aeiou]]</literal>.
6004+
</para>
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</listitem>
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<listitem>
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<para>
6008+
XQuery character class shorthands <literal>\c</literal>,
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<literal>\C</literal>, <literal>\i</literal>,
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and <literal>\I</literal> are not supported.
6011+
</para>
6012+
</listitem>
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<listitem>
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<para>
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XQuery character class elements
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using <literal>\p{UnicodeProperty}</literal> or the
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inverse <literal>\P{UnicodeProperty}</literal> are not supported.
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</para>
6019+
</listitem>
6020+
<listitem>
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<para>
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POSIX interprets character classes such as <literal>\w</literal>
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(see <xref linkend="posix-class-shorthand-escapes-table"/>)
6024+
according to the prevailing locale (which you can control by
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attaching a <literal>COLLATE</literal> clause to the operator or
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function). XQuery specifies these classes by reference to Unicode
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character properties, so equivalent behavior is obtained only with
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a locale that follows the Unicode rules.
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</para>
6030+
</listitem>
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<listitem>
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<para>
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The SQL standard (not XQuery itself) attempts to cater for more
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variants of <quote>newline</quote> than POSIX does. The
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newline-sensitive matching options described above consider only
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ASCII NL (<literal>\n</literal>) to be a newline, but SQL would have
6037+
us treat CR (<literal>\r</literal>), CRLF (<literal>\r\n</literal>)
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(a Windows-style newline), and some Unicode-only characters like
6039+
LINE SEPARATOR (U+2028) as newlines as well.
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Notably, <literal>.</literal> and <literal>\s</literal> should
6041+
count <literal>\r\n</literal> as one character not two according to
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SQL.
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</para>
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</listitem>
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<listitem>
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<para>
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Of the character-entry escapes described in
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<xref linkend="posix-character-entry-escapes-table"/>,
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XQuery supports only <literal>\n</literal>, <literal>\r</literal>,
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and <literal>\t</literal>.
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</para>
6052+
</listitem>
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<listitem>
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<para>
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XQuery does not support
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the <literal>[:<replaceable>name</replaceable>:]</literal> syntax
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for character classes within bracket expressions.
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</para>
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</listitem>
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<listitem>
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<para>
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XQuery does not have lookahead or lookbehind constraints,
6063+
nor any of the constraint escapes described in
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<xref linkend="posix-constraint-escapes-table"/>.
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</para>
6066+
</listitem>
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<listitem>
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<para>
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The metasyntax forms described in <xref linkend="posix-metasyntax"/>
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do not exist in XQuery.
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</para>
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</listitem>
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<listitem>
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<para>
6075+
The regular expression flag letters defined by XQuery are
6076+
related to but not the same as the option letters for POSIX
6077+
(<xref linkend="posix-embedded-options-table"/>). While the
6078+
<literal>i</literal> and <literal>q</literal> options behave the
6079+
same, others do not:
6080+
<itemizedlist>
6081+
<listitem>
6082+
<para>
6083+
XQuery's <literal>s</literal> (allow dot to match newline)
6084+
and <literal>m</literal> (allow <literal>^</literal>
6085+
and <literal>$</literal> to match at newlines) flags provide
6086+
access to the same behaviors as
6087+
POSIX's <literal>n</literal>, <literal>p</literal>
6088+
and <literal>w</literal> flags, but they
6089+
do <emphasis>not</emphasis> match the behavior of
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POSIX's <literal>s</literal> and <literal>m</literal> flags.
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Note in particular that dot-matches-newline is the default
6092+
behavior in POSIX but not XQuery.
6093+
</para>
6094+
</listitem>
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<listitem>
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<para>
6097+
XQuery's <literal>x</literal> (ignore whitespace in pattern) flag
6098+
is noticeably different from POSIX's expanded-mode flag.
6099+
POSIX's <literal>x</literal> flag also
6100+
allows <literal>#</literal> to begin a comment in the pattern,
6101+
and POSIX will not ignore a whitespace character after a
6102+
backslash.
6103+
</para>
6104+
</listitem>
6105+
</itemizedlist>
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</para>
6107+
</listitem>
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</itemizedlist>
6109+
</para>
6110+
6111+
</sect3>
59736112
</sect2>
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</sect1>
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@@ -11793,6 +11932,14 @@ table2-mapping
1179311932
</programlisting>
1179411933
</para>
1179511934
</listitem>
11935+
11936+
<listitem>
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<para>
11938+
There are minor differences in the interpretation of regular
11939+
expression patterns used in <literal>like_regex</literal> filters, as
11940+
described in <xref linkend="jsonpath-regular-expressions"/>.
11941+
</para>
11942+
</listitem>
1179611943
</itemizedlist>
1179711944

1179811945
<sect3 id="strict-and-lax-modes">
@@ -11872,6 +12019,63 @@ table2-mapping
1187212019

1187312020
</sect3>
1187412021

12022+
<sect3 id="jsonpath-regular-expressions">
12023+
<title>Regular Expressions</title>
12024+
12025+
<indexterm zone="jsonpath-regular-expressions">
12026+
<primary><literal>LIKE_REGEX</literal></primary>
12027+
<secondary>in SQL/JSON</secondary>
12028+
</indexterm>
12029+
12030+
<para>
12031+
SQL/JSON path expressions allow matching text to a regular expression
12032+
with the <literal>like_regex</literal> filter. For example, the
12033+
following SQL/JSON path query would case-insensitively match all
12034+
strings in an array that start with an English vowel:
12035+
<programlisting>
12036+
'$[*] ? (@ like_regex "^[aeiou]" flag "i")'
12037+
</programlisting>
12038+
</para>
12039+
12040+
<para>
12041+
The optional <literal>flag</literal> string may include one or more of
12042+
the characters
12043+
<literal>i</literal> for case-insensitive match,
12044+
<literal>m</literal> to allow <literal>^</literal>
12045+
and <literal>$</literal> to match at newlines,
12046+
<literal>s</literal> to allow <literal>.</literal> to match a newline,
12047+
and <literal>q</literal> to quote the whole pattern (reducing the
12048+
behavior to a simple substring match).
12049+
</para>
12050+
12051+
<para>
12052+
The SQL/JSON standard borrows its definition for regular expressions
12053+
from the <literal>LIKE_REGEX</literal> operator, which in turn uses the
12054+
XQuery standard. PostgreSQL does not currently support the
12055+
<literal>LIKE_REGEX</literal> operator. Therefore,
12056+
the <literal>like_regex</literal> filter is implemented using the
12057+
POSIX regular expression engine described in
12058+
<xref linkend="functions-posix-regexp"/>. This leads to various minor
12059+
discrepancies from standard SQL/JSON behavior, which are cataloged in
12060+
<xref linkend="posix-vs-xquery"/>.
12061+
Note, however, that the flag-letter incompatibilities described there
12062+
do not apply to SQL/JSON, as it translates the XQuery flag letters to
12063+
match what the POSIX engine expects.
12064+
</para>
12065+
12066+
<para>
12067+
Keep in mind that the pattern argument of <literal>like_regex</literal>
12068+
is a JSON path string literal, written according to the rules given in
12069+
<xref linkend="datatype-jsonpath"/>. This means in particular that any
12070+
backslashes you want to use in the regular expression must be doubled.
12071+
For example, to match strings that contain only digits:
12072+
<programlisting>
12073+
'$ ? (@ like_regex "^\\d+$")'
12074+
</programlisting>
12075+
</para>
12076+
12077+
</sect3>
12078+
1187512079
<sect3 id="functions-sqljson-path-operators">
1187612080
<title>SQL/JSON Path Operators and Methods</title>
1187712081

@@ -12113,10 +12317,11 @@ table2-mapping
1211312317
<row>
1211412318
<entry><literal>like_regex</literal></entry>
1211512319
<entry>
12116-
Tests pattern matching with POSIX regular expressions
12117-
(see <xref linkend="functions-posix-regexp"/>). Supported flags
12118-
are <literal>i</literal>, <literal>s</literal>, <literal>m</literal>,
12119-
<literal>x</literal>, and <literal>q</literal>.</entry>
12320+
Tests whether the first operand matches the regular expression
12321+
given by the second operand, optionally with modifications
12322+
described by a string of <literal>flag</literal> characters (see
12323+
<xref linkend="jsonpath-regular-expressions"/>)
12324+
</entry>
1212012325
<entry><literal>["abc", "abd", "aBdC", "abdacb", "babc"]</literal></entry>
1212112326
<entry><literal>$[*] ? (@ like_regex "^ab.*c" flag "i")</literal></entry>
1212212327
<entry><literal>"abc", "aBdC", "abdacb"</literal></entry>

‎doc/src/sgml/json.sgml‎

Lines changed: 26 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -666,13 +666,32 @@ SELECT jdoc-&gt;'guid', jdoc-&gt;'name' FROM api WHERE jdoc @&gt; '{"tags": ["qu
666666
</itemizedlist>
667667

668668
<para>
669-
An SQL/JSON path expression is an SQL character string literal,
670-
so it must be enclosed in single quotes when passed to an SQL/JSON
671-
query function. Following the JavaScript
672-
conventions, character string literals within the path expression
673-
must be enclosed in double quotes. Any single quotes within this
674-
character string literal must be escaped with a single quote
675-
by the SQL convention.
669+
An SQL/JSON path expression is typically written in an SQL query as an
670+
SQL character string literal, so it must be enclosed in single quotes,
671+
and any single quotes desired within the value must be doubled
672+
(see <xref linkend="sql-syntax-strings"/>).
673+
Some forms of path expressions require string literals within them.
674+
These embedded string literals follow JavaScript/ECMAScript conventions:
675+
they must be surrounded by double quotes, and backslash escapes may be
676+
used within them to represent otherwise-hard-to-type characters.
677+
In particular, the way to write a double quote within an embedded string
678+
literal is <literal>\"</literal>, and to write a backslash itself, you
679+
must write <literal>\\</literal>. Other special backslash sequences
680+
include those recognized in JSON strings:
681+
<literal>\b</literal>,
682+
<literal>\f</literal>,
683+
<literal>\n</literal>,
684+
<literal>\r</literal>,
685+
<literal>\t</literal>,
686+
<literal>\v</literal>
687+
for various ASCII control characters, and
688+
<literal>\u<replaceable>NNNN</replaceable></literal> for a Unicode
689+
character identified by its 4-hex-digit code point. The backslash
690+
syntax also includes two cases not allowed by JSON:
691+
<literal>\x<replaceable>NN</replaceable></literal> for a character code
692+
written with only two hex digits, and
693+
<literal>\u{<replaceable>N...</replaceable>}</literal> for a character
694+
code written with 1 to 6 hex digits.
676695
</para>
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678697
<para>

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