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From: "David G. Johnston" <david.g.johnston@gmail.com>
Date: Mon, 20 May 2024 20:44:04 -0700
Message-ID: <CAKFQuwYG5+gGjFLoAcYk8BBW4EGY6i-NQG7N2FqxyAp8eunR9A@mail.gmail.com>
Subject: Re: doc regexp_replace replacement string \n does not explained properly
To: jian he <jian.universality@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Monday, May 20, 2024, jian he <jian.universality@gmail.com> wrote:

> hi.
>
> https://www.postgresql.org/docs/current/functions-
> matching.html#FUNCTIONS-POSIX-REGEXP
>
>
>  If there is a match,
> the source string is returned with the replacement string substituted
> for the matching substring.


>
This happens regardless of the presence of parentheses.


>
>  The replacement string can contain \n,
> where n is 1 through 9, to indicate that the source substring matching
> the n'th parenthesized subexpression of the pattern should be
> inserted, and it can contain \& to indicate that the substring
> matching the entire pattern should be inserted.


 Then if the replacement text contains =E2=80=9C\n=E2=80=9D expressions tho=
se are replaced
with text captured from the corresponding parentheses group.


> <<
> i think it explained example like:
> SELECT regexp_replace('foobarbaz', 'b(..)', 'X\1Y', 'g');


global - find two matches to process.

foobarbaz
fooX\1YX\1Y
fooXarYXazY


>
> but it does not seem to explain cases like:
> SELECT regexp_replace('foobarbaz', 'b(..)', 'X\2Y', 'g');
>
>
foobarbaz
fooX\2YX\2Y
fooX{empty string, no second capture group}YX{empty}Y
fooXYXY

The docs are correct, though I suppose being explicit that a missing
capture group results in an empty string substitution instead of an error
is probably warranted.

David J.

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On Monday, May 20, 2024, jian he &lt;<a href=3D"mailto:jian.universality@gm=
ail.com">jian.universality@gmail.com</a>&gt; wrote:<br><blockquote class=3D=
"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding=
-left:1ex">hi.<br>
<br>
<a href=3D"https://www.postgresql.org/docs/current/functions-matching.html#=
FUNCTIONS-POSIX-REGEXP" target=3D"_blank">https://www.postgresql.org/<wbr>d=
ocs/current/functions-<wbr>matching.html#FUNCTIONS-POSIX-<wbr>REGEXP</a><br=
><br></blockquote><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .=
8ex;border-left:1px #ccc solid;padding-left:1ex"><br></blockquote><blockquo=
te class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc so=
lid;padding-left:1ex">=C2=A0If there is a match,<br>
the source string is returned with the replacement string substituted<br>
for the matching substring.</blockquote><blockquote class=3D"gmail_quote" s=
tyle=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>=
</blockquote><div><br></div><div>This happens regardless of the presence of=
 parentheses.</div><div><br></div><blockquote class=3D"gmail_quote" style=
=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br></bl=
ockquote><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;borde=
r-left:1px #ccc solid;padding-left:1ex"><br></blockquote><blockquote class=
=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padd=
ing-left:1ex">=C2=A0The replacement string can contain \n,<br>
where n is 1 through 9, to indicate that the source substring matching<br>
the n&#39;th parenthesized subexpression of the pattern should be<br>
inserted, and it can contain \&amp; to indicate that the substring<br>
matching the entire pattern should be inserted.</blockquote><div><br></div>=
<div>=C2=A0Then if the replacement text contains =E2=80=9C\n=E2=80=9D expre=
ssions those are replaced with text captured from the corresponding parenth=
eses group.</div><div><br></div><blockquote class=3D"gmail_quote" style=3D"=
margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
&lt;&lt;<br>
i think it explained example like:<br>
SELECT regexp_replace(&#39;foobarbaz&#39;, &#39;b(..)&#39;, &#39;X\1Y&#39;,=
 &#39;g&#39;);</blockquote><div><br></div><div>global - find two matches to=
 process.</div><div><br></div><div>foobarbaz</div><div>fooX\1YX\1Y</div><di=
v>fooXarYXazY</div><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=
=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
but it does not seem to explain cases like:<br>
SELECT regexp_replace(&#39;foobarbaz&#39;, &#39;b(..)&#39;, &#39;X\2Y&#39;,=
 &#39;g&#39;);<br><br></blockquote><div><br></div><div>foobarbaz</div><div>=
fooX\2YX\2Y</div><div>fooX{empty string, no second capture group}YX{empty}Y=
</div><div>fooXYXY</div><div><br></div><div>The docs are correct, though I =
suppose being explicit that a missing capture group results in an empty str=
ing substitution instead of an error is probably warranted.</div><div><br><=
/div><div>David J.</div><div><br></div>

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