Regex search string with partial matches allowed

Suppose I have a search pattern like ^FOO.B.A.R, and I want to check whether a string matches the full search pattern, but, if the string is shorter than the search pattern, it’s allowed to match only part of it.

That is:

• If the string is 1 character long, it must match ^F
• If the string is 2 characters long, it must match ^FO
• If the string is 3 characters long, it must match ^FOO
• If the string is 4 characters long, it must match ^FOO.
• …
• If the string is 9 or more characters long, it must match ^FOO.B.A.R

Is there a way to specify this in the regex search pattern, or do I need to detect the length of the string and then build the pattern accordingly?

>Solution :

Probably the simplest way to solve this is to adjust the length of the pattern to match the input string, adding a $ to assert end-of-string for strings less than the length of the regex. You can then use re.match to test. For example:

regex = r'FOO.B.A.R'

strs = ['F', 'FOO', 'FOX', 'FOOYB', 'FOOYA', 'FOOXBYAZR', 'FOOBBAARRX']

for s in strs:
    pattern = regex[:len(s)] + ('$' if len(s) < len(regex) else '')
    print(f'{s}: {re.match(pattern, s) is not None}')

Output:

F: True
FOO: True
FOX: False
FOOYB: True
FOOYA: False
FOOXBYAZR: True
FOOBBAARRX: True