Regex matching between two strings?


Question

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!

1
18
12/24/2016 5:57:30 PM

Accepted Answer

this should do the trick

 m = re.findall ( '<!--(.*?)-->', string, re.DOTALL)
36
10/5/2012 11:55:25 AM

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.


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