python regex capture groups

Question

I'm trying to write a regex pattern which will either match a number or a number and a trailing string. So match should ouput:

Matching "string100":      [('100', '')]
Matching "string900_TYPE": [('900', 'TYPE')]

But instead, I get:

Matching "string100":      [('100', '')]
Matching "string900_TYPE": [('900', ''), ('', 'TYPE')]

The idea is to have the number as the first item in the tuple and the "TYPE" as the second, so I can easily determine whether "TYPE" exists in a tuple or not (second tuple item is empty --> '').

Code:

import re


stringList = ["string100", "string900_TYPE"]
pattern = r"(\d{3})|\w(TYPE)"

for string in stringList:
    match = re.findall(pattern, string)
    print('Matching "' + string + '":\t', match)

Thanks in advance.

Like that: (\d{3})(?:_(\w+))?. Don't use an alternation, describe the full string. — Casimir et Hippolyte
– Casimir et Hippolyte, Commented Oct 9, 2017 at 21:05
The pattern can actually be simplified to +(\d{3})_?(\w+)? which requires less steps for the regex engine. Is there any reason for using a non-capturing group around the subgroup (\w+) ? — J. Doe
– J. Doe, Commented Oct 9, 2017 at 22:45
If you use (\d{3})_?(\w+)? (note there must be no + at the start) then you may also match 123_. (\d{3})(?:_(\w+))? is best here since (?:_(\w+))? makes the whole sequence of patterns optional. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Oct 9, 2017 at 22:46
The + must have slipped in somehow... 123_ does not seem to be captured, though. — J. Doe
– J. Doe, Commented Oct 9, 2017 at 23:02

J. Doe · Accepted Answer · 2017-10-09 23:22:09Z

1

(\d{3})(?:_(\w+))?will do the trick. (Thanks to @Casimir et Hippolyte)

It is also more robust than using a simpler pattern like (\d{3})_?(\w+)? (thanks to @Wiktor Stribiżew)

answered Oct 9, 2017 at 23:22

J. Doe

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