From 1b5d34ca6244a9296215325a9f82fb805e739f9e Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 4 Aug 2015 21:09:12 -0400
Subject: [PATCH] Docs: add an explicit example about controlling overall
greediness of REs.
Per discussion of bug #13538.
---
doc/src/sgml/func.sgml | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index fd82ea4f4e5..59121da5363 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -5203,10 +5203,37 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
The quantifiers <literal>{1,1}</> and <literal>{1,1}?</>
can be used to force greediness or non-greediness, respectively,
on a subexpression or a whole RE.
+ This is useful when you need the whole RE to have a greediness attribute
+ different from what's deduced from its elements. As an example,
+ suppose that we are trying to separate a string containing some digits
+ into the digits and the parts before and after them. We might try to
+ do that like this:
+<screen>
+SELECT regexp_matches('abc01234xyz', '(.*)(\d+)(.*)');
+<lineannotation>Result: </lineannotation><computeroutput>{abc0123,4,xyz}</computeroutput>
+</screen>
+ That didn't work: the first <literal>.*</> is greedy so
+ it <quote>eats</> as much as it can, leaving the <literal>\d+</> to
+ match at the last possible place, the last digit. We might try to fix
+ that by making it non-greedy:
+<screen>
+SELECT regexp_matches('abc01234xyz', '(.*?)(\d+)(.*)');
+<lineannotation>Result: </lineannotation><computeroutput>{abc,0,""}</computeroutput>
+</screen>
+ That didn't work either, because now the RE as a whole is non-greedy
+ and so it ends the overall match as soon as possible. We can get what
+ we want by forcing the RE as a whole to be greedy:
+<screen>
+SELECT regexp_matches('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
+<lineannotation>Result: </lineannotation><computeroutput>{abc,01234,xyz}</computeroutput>
+</screen>
+ Controlling the RE's overall greediness separately from its components'
+ greediness allows great flexibility in handling variable-length patterns.
</para>
<para>
- Match lengths are measured in characters, not collating elements.
+ When deciding what is a longer or shorter match,
+ match lengths are measured in characters, not collating elements.
An empty string is considered longer than no match at all.
For example:
<literal>bb*</>
--
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