python - Regex matching between two strings?

18
votes

I can't seem to find a way to extract all comments like in following example.

>>> import re
>>> string = '''
... <!-- one 
... -->
... <!-- two -- -- -->
... <!-- three -->
... '''
>>> m = re.findall ( '<!--([^\(-->)]+)-->', string, re.MULTILINE)
>>> m
[' one \n', ' three ']

block with two -- -- is not matched most likely because of bad regex. Can someone please point me in right direction how to extract matches between two strings.

Hi I've tested what you guys suggested in comments.... here is working solution with little upgrade.

>>> m = re.findall ( '<!--(.*?)-->', string, re.MULTILINE)
>>> m
[' two -- -- ', ' three ']
>>> m = re.findall ( '<!--(.*\n?)-->', string, re.MULTILINE)
>>> m
[' one \n', ' two -- -- ', ' three ']

thanks!

2
pythonregexpython-3.xregex-negation
anything between the [] is a single character so (-->) will not look for that grouping is part of the problem... – Joran Beasley
re.findall('<!--(.*?)-->', string, re.DOTALL) should do. You don't need ^\(-->) here, because the question mark makes it non-greedy. – BrtH
You look like you're looking for just the words? If so, what's wrong with m = re.findall('[\w]+', string, re.MULTILINE)? Also, string is a really bad name for a, um, string. – Ben

2 Answers

38
votes

this should do the trick

 m = re.findall ( '<!--(.*?)-->', string, re.DOTALL)
3
votes

In general, it is impossible to do arbitrary matching between two delimiters with a regular grammar.

Specifcally, if you allow nesting, 

<!-- how do you deal <!-- with nested --> comments? -->

you'll run in to issues. So, while you may be able to solve this specific problem with a regular expression, any regular expression that you write will be able to be broken by some other strange nesting of comments.

To parse arbitrary comments, you'll need to move on to a method of parsing context free grammars. A simple method to do so is to use a pushdown automaton.