why does this python regex fail

Question

import sys
import os
import re
import numpy as np
#Tags to remove, sample line:  1:one:2:two:....:122:twentytwo:....:194:ninetyfour:....
r122 = re.compile(':122:(.):')
r194  = re.compile(':194:(.):')

if len(sys.argv) < 2 :
    sys.exit('Usage: python %s <file2filter>' % sys.argv[0])
if not os.path.exists(sys.argv[1]):
    sys.exit('ERROR: file %s not found!' % sys.argv[1])
with open (sys.argv[1]) as f:
    for line in f:
        line = re.sub(r':122:(.):', '', str(line))
        line = re.sub(r':194:(.):', '', str(line))
        print(line,end=" ")

In

1:one:2:two:....:122:twentytwo:....:194:ninetyfour:....

Out

1:one:2:two:....:122:twentytwo:....:194:ninetyfour:....

the tags 122 and 194 are not removed. what am i missing here ?

I want to remove :122:twentytwo: and :194:ninetyfour: from the lines in the file — user1409254
– user1409254, Commented Apr 30, 2020 at 17:31
So, you need to replace (.) with [^:]+ in your patterns. And you need just one, with open (sys.argv[1], 'r') as f: and then for line in f: print(re.sub(r':1(?:22|94):[^:]+:', '', line)) — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Apr 30, 2020 at 17:33
(.) only matches one character between :122: and :, but twentytwo is longer than 1 character. — Barmar
– Barmar, Commented Apr 30, 2020 at 17:33

Wiktor Stribiżew · Accepted Answer · 2020-04-30 18:01:53Z

Your patterns contain (.) that matches and captures any single char other than a line break char. What you want is to match any chars other than :, so you need to use [^:]+.

You do not need to compile separate regex objects if only a part of your regex changes. You may build you regex dynamically abd compile once before reading the file. E.g. you have 122, 194 and 945 values to use in :...:[^:]+: pattern in place of ..., then you may use

vals = ["122", "194", "945"]
r = re.compile(r':(?:{}):[^:]+:'.format("|".join(vals)))
# Or, using f-strings
# r = re.compile(rf':(?:{"|".join(vals)}):[^:]+:')

The regex will look like :(?:122|194|945):[^:]+::

: - a colon
(?:122|194|945) - a non-capturing group matching 122, 194 or 945
: - a colon
[^:]+ - 1+ chars other than a :
: - a colon

Then use

with open (sys.argv[1], 'r') as f: 
    for line in f:
        print(r.sub('', line))

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