Some input chars (i.e. '++') break re.match

[jpfisher]

BPO	22119
Nosy	@ezio-melotti

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-08-01.21:27:18.635>
created_at = <Date 2014-08-01.20:44:52.594>
labels = ['expert-regex', 'type-bug', 'invalid']
title = "Some input chars (i.e. '++') break re.match"
updated_at = <Date 2014-08-01.21:27:18.634>
user = 'https://bugs.python.org/jpfisher'

bugs.python.org fields:

activity = <Date 2014-08-01.21:27:18.634>
actor = 'ezio.melotti'
assignee = 'none'
closed = True
closed_date = <Date 2014-08-01.21:27:18.635>
closer = 'ezio.melotti'
components = ['Regular Expressions']
creation = <Date 2014-08-01.20:44:52.594>
creator = 'jpfisher'
dependencies = []
files = []
hgrepos = []
issue_num = 22119
keywords = []
message_count = 2.0
messages = ['224518', '224521']
nosy_count = 3.0
nosy_names = ['ezio.melotti', 'mrabarnett', 'jpfisher']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22119'
versions = ['Python 2.7']

[jpfisher]

Some characters repeated in the pattern break re.match:

Linux python 2.7.6
###################################
# test.py
import re

#diffitem = "libstdc+"   succeeds
#diffitem = "libstdc++"  fails
#diffitem = "libstdc**"  fails
#diffitem = "libstdc.."  succeeds
diffitem = "libstdc+\+"  succeeds
line = "time  1.7-23build1"

result = re.match(diffitem, line)
print result

###################################
$ python  test.py
Traceback (most recent call last):
  File "test.py", line 9, in <module>
    result = re.match(diffitem, line)
  File "/usr/lib/python2.7/re.py", line 137, in match
    return _compile(pattern, flags).match(string)
  File "/usr/lib/python2.7/re.py", line 244, in _compile
    raise error, v # invalid expression
sre_constants.error: multiple repeat

[mrabarnett]

In a regex, '+' is a metacharacter meaning "repeated one or more times".

"libstdc+" will match "libstd" followed by "c" repeated one or more times.

"libstdc++" will match "libstd" followed by "c" repeated one or more times, but then there's another "+", which it takes to mean that you want the repeat to be repeated, hence the exception.

'*' is also a metacharacter, this one meaning "repeated zero or more times".

In summary, not a bug.