New submission from Philippe Verdy <verd...@wanadoo.fr>:
For now, when capturing groups are used within repetitions, it is impossible to
capure what they match
individually within the list of matched repetitions.
E.g. the following regular expression:
(0|1[0-9]{0,2}|2(?:[0-4][0-9]?|5[0-5]?)?)(?:\.(0|1[0-9]{0,2}|2(?:[0-4][0-9]?|5[0-5]?)?)){3}
is a regexp that contains two capturing groups (\1 and \2), but whose the
second one is repeated (3 times) to
match an IPv4 address in dotted decimal format. We'd like to be able to get the
individual multiple matchs
for the second group.
For now, capturing groups don't record the full list of matches, but just
override the last occurence of the
capturing group (or just the first if the repetition is not greedy, which is
not the case here because the
repetition "{3}" is not followed by a "?"). So \1 will effectively return the
first decimal component of the
IPv4 address, but \2 will just return the last (fourth) decimal component.
I'd like to have the possibility to have a compilation flag "R" that would
indicate that capturing groups
will not just return a single occurence, but all occurences of the same group.
If this "R" flag is enabled,
then:
- the Match.group(index) will not just return a single string but a list of
strings, with as many occurences
as the number of effective repetitions of the same capturing group. The last
element in that list will be the
one equal to the current behavior
- the Match.start(index) and Match.end(index) will also both return a list of
positions, those lists having
the same length as the list returned by Match.group(index).
- for consistency, the returned values as lists of strings (instead of just
single strings) will apply to all
capturing groups, even if they are not repeated.
Effectively, with the same regexp above, we will be able to retreive (and
possibily substitute):
- the first decimal component of the IPv4 address with "{\1:1}" (or "{\1:}" or
"{\1}" or "\1" as before),
i.e. the 1st (and last) occurence of capturing group 1, or in
Match.group(1)[1], or between string positions Match.start(1)[1] and
Match.end(1)[1] ;
- the second decimal component of the IPv4 address with "{\2:1}", i.e. the 1st
occurence of capturing group
2, or in Match.group(2)[1], or between string positions Match.start(2)[1] and
Match.end(2)[1] ;
- the third decimal component of the IPv4 address with "{\2:2}", i.e. the 2nd
occurence of capturing group 2,
or in Match.group(2)[2], or between string positions Match.start(2)[2] and
Match.end(2)[2] ;
- the fourth decimal component of the IPv4 address with "{\2:3}" (or "{\2:}" or
"{\2}" or "\2"), i.e. the 3rd
(and last) occurence of capturing group 2, or in Match.group(2)[2], or between
string positions
Match.start(2)[3] and Match.end(2)[3] ;
This should work with all repetition patterns (both greedy and not greedy,
atomic or not, or possessive), in
which the repeated pattern contains any capturing group.
This idea should also be submitted to the developers of the PCRE library (and
Perl from which they originate,
and PHP where PCRE is also used), so that they adopt a similar behavior in
their regular expressions.
If there's already a candidate syntax or compilation flag in those libraries,
this syntax should be used for
repeated capturing groups.
----------
components: Library (Lib)
messages: 94022
nosy: verdy_p
severity: normal
status: open
title: Regexp: capturing groups in repetitions
type: feature request
versions: Python 3.2
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue7132>
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