Lookahead and Lookbehind
Zero-width assertions that match based on surrounding context without consuming characters. Positive lookahead: (?=...). Negative: (?!...). Lookbehind: (?<=...).
Python
Advanced
5 min read
Lookahead and Lookbehind
Example
import re
text = "100px 200em 300px"
# Find numbers followed by "px"
matches = re.findall(r"\d+(?=px)", text)
print(matches) # ["100", "300"]
# Find numbers NOT followed by "px"
matches2 = re.findall(r"\d+(?!px)", text)
print(matches2) # ["200"] Pro Tip
Lookbehind assertions must be fixed-width in Python.