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Doc: improve documentation around jsonpath regular expressions.

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Provide some documentation about the differences between XQuery
regular 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 who
has to translate a LIKE_REGEX query to Postgres.  (Eventually we might
extend Spencer's engine to allow precise implementation of XQuery,
but not today.)

Reference that in the jsonpath docs, provide definitions of the
XQuery flag letters, and add a description of the JavaScript-inspired
string literal syntax used within jsonpath.  Also point out explicitly
that backslashes used within like_regex patterns will need to be doubled.

This also syncs the docs with the decision implemented in commit
d5b90cd64 to desupport XQuery's 'x' flag for now.

Jonathan Katz and Tom Lane

Discussion: https://postgr.es/m/CAPpHfdvDci4iqNF9fhRkTqhe-5_8HmzeLt56drH%2B_Rv2rNRqfg@mail.gmail.com
This commit is contained in:
Tom Lane
2019-09-19 11:22:21 -04:00
parent e1c8743e6c
commit 0a97edb12e
2 changed files with 235 additions and 11 deletions
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33
doc/src/sgml/json.sgml
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@ -666,13 +666,32 @@ SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @> '{"tags": ["qu
</itemizedlist>
<para>
An SQL/JSON path expression is an SQL character string literal,
so it must be enclosed in single quotes when passed to an SQL/JSON
query function. Following the JavaScript
conventions, character string literals within the path expression
must be enclosed in double quotes. Any single quotes within this
character string literal must be escaped with a single quote
by the SQL convention.
An SQL/JSON path expression is typically written in an SQL query as an
SQL character string literal, so it must be enclosed in single quotes,
and any single quotes desired within the value must be doubled
(see <xref linkend="sql-syntax-strings"/>).
Some forms of path expressions require string literals within them.
These embedded string literals follow JavaScript/ECMAScript conventions:
they must be surrounded by double quotes, and backslash escapes may be
used within them to represent otherwise-hard-to-type characters.
In particular, the way to write a double quote within an embedded string
literal is <literal>\"</literal>, and to write a backslash itself, you
must write <literal>\\</literal>. Other special backslash sequences
include those recognized in JSON strings:
<literal>\b</literal>,
<literal>\f</literal>,
<literal>\n</literal>,
<literal>\r</literal>,
<literal>\t</literal>,
<literal>\v</literal>
for various ASCII control characters, and
<literal>\u<replaceable>NNNN</replaceable></literal> for a Unicode
character identified by its 4-hex-digit code point. The backslash
syntax also includes two cases not allowed by JSON:
<literal>\x<replaceable>NN</replaceable></literal> for a character code
written with only two hex digits, and
<literal>\u{<replaceable>N...</replaceable>}</literal> for a character
code written with 1 to 6 hex digits.
</para>
<para>