CODEWITHSUNDEEP
pythonregex
Jaroslav Bezděk
Jaroslav Bezděk

Reputation: 7625

Python: regex pattern to extract substring after prefix when no suffix is present

I would like to extract substring from every item of a list. The substring has to be placed after 'opt_' prefix and no '_join' suffix can be present in a string.

My input:

my_opts = [
    'opt_tw',
    'opt_ls_join',
    'opt_ac_join',
    'opt_pan_join',
    'opt_full_led',
]

Desired output:

['tw', 'full_led']

What I have tried:

>>> import re
>>> pattern = r'opt_?(.*)[^_join]'
>>> print([
...     re.search(pattern, opt).group(1)
...     for opt in my_opts
...     if re.match(pattern, opt)
... ])
['t', 'l', 'a', 'p', 'full_le']

Can you help me, please?

Upvotes: 1

Views: 1857

Answers (2)

The fourth bird
The fourth bird

Reputation: 163342

You can match _opt and optionally match until the last occurrence of _.

Then assert not join at the end of the string, and capture the rest in group 1.

opt_((?:.*_)?(?!join$)[^\r\n_]+)$
  • opt_ Match literally
  • ( Capture group 1
    • (?:.*_)? Optionally match until the last occurrence of _
    • (?!join$) Negative lookahead, assert not join at the end of the string
    • [^\r\n_]+ Match 1+ times any char except _ (or newlines)
  • ) Close group 1
  • $ End of string

REgex demo

import re

my_opts = [
    'opt_tw',
    'opt_ls_join',
    'opt_ac_join',
    'opt_pan_join',
    'opt_full_led',
]

pattern = r"opt_((?:.*_)?(?!join$)[^\r\n_]+)$"
for s in my_opts:
    match = re.match(pattern, s)
    if match:
        print(match.group(1))

Output

tw
full_led

If the string should not contain _join you can use a negative lookahead

^opt_(?!.*_join)(.+)

Regex demo

Upvotes: 3

Mustafa Aydın
Mustafa Aydın

Reputation: 18306

You can use str.startswith and str.endswith for the conditions and string slicing for the capturing group:

out = [opt[4:]
       for opt in my_opts
       if opt.startswith("opt_") and not opt.endswith("_join")]

where 4 is equal to the length of "opt_" and helps get substring after that,

to get

>>> out
["tw", "full_led"]

Upvotes: 2

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