python-regexHow to use negative lookbehind in Python regex?

Negative lookbehind is a feature of regular expressions that allows you to match a pattern only if it is not preceded by another pattern. It is supported in Python through the re module.

Example

import re

# Match any word that is not preceded by "not"
pattern = r"(?<!not)\b\w+\b"

string = "This is not a test"

match = re.search(pattern, string)

if match:
    print(match.group())

Output example

This

Code explanation

• (?<!not) - Negative lookbehind assertion that matches only if the pattern is not preceded by "not"
• \b\w+\b - Matches any word
• re.search(pattern, string) - Searches for the pattern in the string
• match.group() - Returns the matched pattern

Helpful links

• Python Regular Expression Documentation
• Negative Lookbehind Tutorial

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