Question

I want to search for a regex match in a larger string from a certain position onwards, and without using string slices.

My background is that I want to search through a string iteratively for matches of various regex’s. A natural solution in Python would be keeping track of the current position within the string and using e.g.

re.match(regex, largeString[pos:])

in a loop. But for really large strings (~ 1MB) string slicing as in largeString[pos:] becomes expensive. I’m looking for a way to get around that.

Side note: Funnily, in a niche of the Python documentation, it talks about an optional pos parameter to the match function (which would be exactly what I want), which is not to be found with the functions themselves :-).

Answer 1

The variants with pos and endpos parameters only exist as members of regular expression objects. Try this:

import re
pattern = re.compile("match here")
input = "don't match here, but do match here"
start = input.find(",")
print pattern.search(input, start).span()

… outputs (25, 35)