Regex search string with partial matches allowed

Question

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?

Asked By: LarrySnyder610

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Source

Answer 1

There is no way to specify this directly in the regex search pattern. However, you can generate the regex pattern dynamically based on the length of the string using a function or code. For example, in JavaScript, you could generate the regex pattern like this:

function generateRegexPattern(str) {
  const length = str.length;
  let pattern = "^";
  for (let i = 0; i < length; i++) {
    pattern += str.charAt(i);
    if (i < length - 1) {
      pattern += ".";
    }
  }
  pattern += "$";
  return new RegExp(pattern);
}

This function takes a string as input and generates a regex pattern that matches the full search pattern if the string is long enough, or matches only part of the search pattern if the string is shorter.

Answered By: Tolga Yolal

Answer 2

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

Answered By: Nick

Regex search string with partial matches allowed

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