How to search for the last occurrence of a regular expression in a string in python?

Question

In python, I can easily search for the first occurrence of a regex within a string like this:

import re
re.search("pattern", "target_text")

Now I need to find the last occurrence of the regex in a string, this doesn't seems to be supported by re module.

I can reverse the string to "search for the first occurrence", but I also need to reverse the regex, which is a much harder problem.

I can also iterate to find all occurrences from left to right, and just keep the last one, but that looks awkward.

Is there a smart way to find the rightmost occurrence?

Answer 1

One approach is to prefix the regex with (?s:.*) and force the engine to try matching at the furthest position and gradually backing off:

re.search("(?s:.*)pattern", "target_text")

Do note that the result of this method may differ from re.findall("pattern", "target_text")[-1] , since the findall method searches for non-overlapping matches, and not all substrings which can be matched are included in the result.

For example, executing the regex aa on abaca , findall would return aba as the only match and select it as the last match, while the code above will return aca as the match.

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Yet another alternative is to use regex package, which supports REVERSE matching mode.

The result would be more or less the same as the method with (?s:.*) in re package as described above. However, since I haven't tried the package myself, it's not clear how backreference works in REVERSE mode - the pattern might require modification in such cases.

Answer 2

import re
re.search("pattern(?!.*pattern)", "target_text")

or

import re
re.findall("pattern", "target_text")[-1]

You can use these 2 approaches.

If you want positions use

x="abc abc abc"
print [(i.start(),i.end(),i.group()) for i in re.finditer(r"abc",x)][-1]

Answer 3

One approach is to use split. For example if you wanted to get the last group after ':' in this sample string:

mystr = 'dafdsaf:ewrewre:cvdsfad:ewrerae'
':'.join(mystr.split(':')[-1:])