How Can I Safely Use User-Supplied Strings as Regex Patterns in Python?

Escaping Regex Characters in User-Supplied Patterns

When utilizing user input as regular expression patterns, it's crucial to address the issue of characters that possess special meanings within regex syntax. For instance, the user's intention to search for "Word (s)" will be misinterpreted as a group by the regex engine due to the parentheses. To prevent this, we need to treat the parentheses as literal strings, not regex symbols.

Conventional Approach: Manual Replacement

One method to escape these characters is to manually replace each instance with its escaped version. For example, we could replace "(s)" with "(s)". However, this requires considering every possible regex symbol, which can be laborious.

A Better Solution: re.escape Function

A more efficient solution is to employ Python's re.escape() function, which automatically escapes non-alphanumeric characters in a string. This allows us to treat special regex characters as literal strings.

For example, to search for any instance of "Word (s)" in a text, we can use:

def simplistic_plural(word, text):
    word_or_plural = re.escape(word) + 's?'
    return re.match(word_or_plural, text)

This function returns a match object if the pattern is found in the text.

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