Why does python's re.search method hang?

Question

I\'m using python regex library to parse some strings and currently I found that my regex is either too complicated or the string I\'m searching is too long.

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Answer 1

The reason why the code execution hangs is catastrophic backtracking due to one obligatory and 1+ optional patterns (those that can match an empty string) inside a quantified group (\w+'?\s*)+ that allows a regex engine to test a lot of matching paths, so many that it takes too long to complete.

I suggest unwrapping the problematic group in such a way that ' or \s become obligatory and wrap them in an optional group:

(\w+(?:['\s]+\w+)*)\s*[-~]\s*(\$?\d+(?:\.\d+)?\$?)
^^^^^^^^^^^^^^^^^^^***

See the regex demo

Here, (\w+(?:['\s]+\w+)*) will match 1+ word chars, and then 0+ sequences of 1+ ' or whitespaces followed with 1+ word chars. This way, the pattern becomes linear and the regex engine fails the match quicker if a non-matching string occurs.

The rest of the pattern:

• \s*[-~]\s* - either - or ~ wrapped with 0+ whitespaces
• (\$?\d+(?:\.\d+)?\$?) - Group 2 capturing
  • \$? - 1 or 0 $ symbols
  • \d+ - 1+ digits
  • (?:\.\d+)? - 1 or 0 zero sequences of:
    • \. - a dot
    • \d+ - 1+ digits
  • \$? - 1 or 0 $ symbols