regexes: How to access multiple matches of a group?
Question:
I am putting together a fairly complex regular expression. One part of the expression matches strings such as ‘+a’, ‘-57’ etc. A + or a – followed by any number of letters or numbers. I want to match 0 or more strings matching this pattern.
This is the expression I came up with:
([+-][a-zA-Z0-9]+)*
If I were to search the string ‘-56+a’ using this pattern I would expect to get two matches:
+a and -56
However, I only get the last match returned:
>>> m = re.match("([+-][a-zA-Z0-9]+)*", '-56+a')
>>> m.groups()
('+a',)
Looking at the python docs I see that:
If a group matches multiple times, only the last match is accessible:
>>> m = re.match(r"(..)+", "a1b2c3") # Matches 3 times.
>>> m.group(1) # Returns only the last match.
'c3'
So, my question is: how do you access multiple group matches?
Answers:
Drop the * from your regex (so it matches exactly one instance of your pattern). Then use either re.findall(...) or re.finditer (see here) to return all matches.
Update:
It sounds like you’re essentially building a recursive descent parser. For relatively simple parsing tasks, it is quite common and entirely reasonable to do that by hand. If you’re interested in a library solution (in case your parsing task may become more complicated later on, for example), have a look at pyparsing.
The regex module fixes this, by adding a .captures method:
>>> m = regex.match(r"(..)+", "a1b2c3")
>>> m.captures(1)
['a1', 'b2', 'c3']
I am putting together a fairly complex regular expression. One part of the expression matches strings such as ‘+a’, ‘-57’ etc. A + or a – followed by any number of letters or numbers. I want to match 0 or more strings matching this pattern.
This is the expression I came up with:
([+-][a-zA-Z0-9]+)*
If I were to search the string ‘-56+a’ using this pattern I would expect to get two matches:
+a and -56
However, I only get the last match returned:
>>> m = re.match("([+-][a-zA-Z0-9]+)*", '-56+a')
>>> m.groups()
('+a',)
Looking at the python docs I see that:
If a group matches multiple times, only the last match is accessible:
>>> m = re.match(r"(..)+", "a1b2c3") # Matches 3 times. >>> m.group(1) # Returns only the last match. 'c3'
So, my question is: how do you access multiple group matches?
Drop the * from your regex (so it matches exactly one instance of your pattern). Then use either re.findall(...) or re.finditer (see here) to return all matches.
Update:
It sounds like you’re essentially building a recursive descent parser. For relatively simple parsing tasks, it is quite common and entirely reasonable to do that by hand. If you’re interested in a library solution (in case your parsing task may become more complicated later on, for example), have a look at pyparsing.
The regex module fixes this, by adding a .captures method:
>>> m = regex.match(r"(..)+", "a1b2c3")
>>> m.captures(1)
['a1', 'b2', 'c3']