From: Tom Lane <tgl@sss.pgh.pa.us>
To: jian he <jian.universality@gmail.com>
cc: pgsql-docs@lists.postgresql.org
Subject: Re: substring synopsis section,
 third argument is optional doc didn't show that
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Comments: In-reply-to jian he <jian.universality@gmail.com>
	message dated "Tue, 21 Jan 2025 16:00:21 +0800"
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Date: Tue, 21 Jan 2025 11:53:24 -0500
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jian he <jian.universality@gmail.com> writes:
> https://www.postgresql.org/docs/current/functions-matching.html#FUNCTION=
S-SIMILARTO-REGEXP

> """
> or as a plain three-argument function:
> substring(string, pattern, escape-character)
> """

> but here "escape-character" is optional.

> substring(string, pattern [,escape-character])
> would be more accurate.

No, the text is correct as written.  substring(text, text) is a
completely different function that implements POSIX regular
expressions, not SQL regular expressions.  It's described in
the next section (9.7.3).  For example,

regression=3D# select substring('foobar', 'o.b');
 substring =

-----------
 oob
(1 row)

regression=3D# select substring('foobar', 'o.b', '');
 substring =

-----------
 =

(1 row)

because '.' is a metacharacter in POSIX but not SQL regexps.

			regards, tom lane