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Docs: add an explicit example about controlling overall greediness of REs.

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Per discussion of bug #13538.
This commit is contained in:
Tom Lane 2015-08-04 21:09:12 -04:00
parent 3bdd7f90fc
commit 1b5d34ca62
1 changed files with 28 additions and 1 deletions
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doc/src/sgml/func.sgml
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@ -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*</>