How Can I Safely Use User Input as a Regular Expression Pattern in Python?

Handling Regex Escape Characters for User Input

When utilizing user input as a regex pattern for a text search, it's crucial to consider characters that hold special significance in regex syntax. Leaving them unhandled can lead to unintended behavior, such as treating '(' and ')' in "Word (s)" as a regex group instead of literal strings.

To effectively handle such cases, the re.escape() function provides a convenient solution. This function escapes non-alphanumeric characters, essentially treating them as literal strings within the regex pattern. By utilizing re.escape(), you can avoid complex replacements for individual regex symbols.

Implementation Example:

An illustrative example is the simplistic_plural() function, which searches for a specified word optionally followed by 's' in a given text:

import re

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

In this function, the word is escaped using re.escape() before constructing the regex pattern. This ensures that any special regex characters within the word are treated as literals, allowing the function to accurately match "Word (s)" as a string rather than a regex group.

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