Python 2.7 regex with format multiple list items

Question

Imagine I want to find all time expressions referring to 'AM' and 'PM' in a string. Let's ignore for the moment that I could use '[AP]M' to do this (because I'm actually pulling the list of valid strings ['AM','PM'] from a dictionary whose keys are language codes). I'd like to look for both at once, like this:

foo = ['am','pm']
separator = ':'
timex = re.compile('(1[012]|[1-9])%s([0-5][0-9])( %s)?' % (separator, foo), re.I)

bar = "It's 6:00 pm, do you know where your brain is?"

timex as written above doesn't get me what I'm after: it only matches to the 'p' in 'pm'. (It seems to be treating all the chars of the list elements as though they were [ampm].)

What I don't want is to do two passes over the string (one each for 'am' and 'pm').

Is there a nice Pythonic way to do a single pass for every item in foo?

Is there a reason why you can't do a check for (am|pm) in regex? — Joe
– Joe, Commented Mar 10, 2014 at 23:10
I'd like to avoid locale-specific expressions. English uses am/pm, but Korean uses 오전/오후, Greek uses π.μ./μ.μ., etc. I tried to simplify the code in my question for readability. In actuality, foo would be something like ampms[locale], where ampms is a dictionary of locales to 2-element lists. — dbl
– dbl, Commented Mar 10, 2014 at 23:17
Did you try joining foo with | and then sticking it in the regex group then? — Joe
– Joe, Commented Mar 10, 2014 at 23:20

Aaron Hall · Accepted Answer · 2014-03-10 23:35:01Z

1

Here's the way I've inserted a list of arbitrary regex terms to be searched for:

import re

foo = ['am','pm']
timex = re.compile('({foo})'.format(foo='|'.join(foo)))

bar = "It's 6:00 pm, do you know where your brain is?"

timex.findall(bar)

returns

['pm']

You can capture more:

>>> timex = re.compile(r'(\d{{1,2}}:\d{{2}})\s*({foo})'.format(foo='|'.join(foo)))
>>> timex.findall(bar)
[('6:00', 'pm')]

edited Mar 10, 2014 at 23:35

answered Mar 10, 2014 at 23:20

Aaron Hall♦

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1 Comment

dbl Over a year ago

Here it is reworked to be closer to the original: timex = re.compile(r'(1[012]|[1-9]){}([0-5][0-9])\s*({})'.format(sep, '|'.join(foo)))

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