python regex problem extract numbers from days hours minutes

Question

I was learning python regex and wondering how to extract numbers from x days y hours z minutes?

Note: there are no months or seconds, only one or more of days, minutes and seconds are allowed.

My attempt

import re

s1 = '5 days 19 hours 30 minutes'
s2 = '5 days'
s3 = '19 hours'
s4 = '5 days 19 hours'

pat = r'((\d+)(?<=\sdays))?((\d+)(?<=\shours))?((\d+)(?<=\sminutes))?'


d,h,m = re.findall(pat,s)

Note: 2 days 3 hours ==> d=2 h=3
      2 hours 3 minutes ==> h=2 m=3

I am struggling to fix lookbehinds. How to fix the problem?

Lucas Vazquez · Accepted Answer · 2020-04-22 00:43:03Z

4

why you add ?<= ? Look, I add groups to your regex and add missing space separations

Then you can match for your regex and select groups.

Python 3.7

import re

s4 = '5 days 19 hours'
pat = r'(?P<days>(\d+)(\sdays))? ?(?P<hours>(\d+)(\shours))? ?(?P<minutes>(\d+)(\sminutes))?'

match = re.match(pat, s4)
if match:
    print(match.groupdict())  # print all groups

# Output: {'days': '5 days', 'hours': '19 hours', 'minutes': None}

If you only want to match the number of the values, instead the name and the number, you need to use the next pattern:

r'((?P<days>\d+) days)? ?((?P<hours>\d+) hours)? ?((?P<minutes>\d+) minutes)?'

"""
Here I deconstruct the pattern,
then you can look at it and the next time you can make your own without help.

((?P<days>\d+) days)?          Match numbers + space + "days"
 ?                             Match space
((?P<hours>\d+) hours)?        Match numbers + space + "hours"
 ?                             Match space
((?P<minutes>\d+) minutes)?    Match numbers + space + "minutes"

If you want the group "days" return you the number and the word "days" yo need to use it as:
(?P<days>\d+ days)
"""

https://regex101.com/ is a good place to try your patterns. It has a good IDE that helps you to understand what each element do.

edited Apr 22, 2020 at 0:43

answered Apr 21, 2020 at 23:57

Lucas Vazquez

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2 Comments

Lucas Vazquez Over a year ago

the walrus operator of python 3.8 allows you to avoid doing a = True; if a: print(a). With it you can do if a:= True: print(a). You can declare a variable inside a conditional! There's a good guide: realpython.com/lessons/assignment-expressions

BhishanPoudel Over a year ago

Thanks a lot, is there a way to just extract numbers? eg. match['days'] = 5 instead of '5 days'

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