How to input a variable string into re.search in python

Question

Initially I had my date regex working as follows, to capture "February 12, 2018" for example

match = re.search(r'(January|February|March|April|May|June|July|August|September?|October?|November|December)\s+\d{1,2},\s+\d{4}', date).group()

But I want it to become more flexible, and input my variable string into my regex but I can't seem to get it to work after looking through many of the stackoverflow threads about similar issues. I'm quite a novice so I'm not sure what's going wrong. I'm aware that simply MONTHS won't work. Thank you

MONTHS = "January|February|March|April|May|June|July|August|September|October|November|December"

match = re.search(r'(MONTHS)\s+\d{1,2},\s+\d{4}', date).group()

print(match)
'NoneType' object has no attribute 'group'

Ah whoops that was from my old string when I had Sep(tember)?, Oct(ober)?, etc. — HenryAD, Aug 13 '18 at 19:01

score 1 · Answer 1 · answered Aug 13 '18 at 18:57

1

You've got MONTHS as just a part of the match string, python doesn't know that it's supposed to be referencing a variable that's storing another string.

So instead, try:

match = re.search(r'(' + MONTHS + ')\s+\d{1,2},\s+\d{4}', date).group()

That will concatenate (stick together) three strings, the first bit, then the string stored in your MONTHS variable, and then the last bit.

answered Aug 13 '18 at 18:57

Steve Archer

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now it works, thank you very much! – HenryAD Aug 13 '18 at 18:58

score 0 · Answer 2 · answered Aug 13 '18 at 19:04

If you want to substitute something into a string, you need to use either format strings (whether an f-string literal or the format or format_map methods on string objects) or printf-style formatting (or template strings, or a third-party library… but usually one of the first two).

Normally, format strings are the easiest solution, but they don't play nice with strings that need braces for other purposes. You don't want that {4} to be treated as "fill in the 4th argument", and escaping it as {{4}} makes things less readable (and when you're dealing with regular expressions, they're already unreadable enough…).

So, printf-style formatting is probably a better option here:

pattern = r'(%s)\s+\d{1,2},\s+\d{4}' % (MONTHS,)

… or:

pattern = r'(%(MONTHS)s)\s+\d{1,2},\s+\d{4}' % {'MONTHS': MONTHS}

How to input a variable string into re.search in python

2 Answers2