Regex Deep Dive

Introduction

Advanced regular expression techniques for complex text processing tasks.

Lookahead and Lookbehind

import re

# Positive lookahead
pattern = r"\d+(?=px)"   # Digits followed by px
re.findall(pattern, "100px 200px 300em")

# Negative lookahead
pattern = r"\d+(?!px)"   # Digits NOT followed by px
re.findall(pattern, "100px 200px 300em")

# Positive lookbehind
pattern = r"(?<=\$)\d+"  # Digits preceded by $
re.findall(pattern, "$100 $200 $300")

# Negative lookbehind
pattern = r"(?<!\$)\d+"  # Digits NOT preceded by $

Named Groups

pattern = r"(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})"
match = re.match(pattern, "2024-06-15")
print(match.group("year"))   # 2024
print(match.groupdict())     # {'year': '2024', 'month': '06', 'day': '15'}

Greedy vs Non-Greedy

# Greedy (matches as much as possible)
re.findall(r"<.+>", "<a><b>")  # ['<a><b>']

# Non-greedy (matches as little as possible)
re.findall(r"<.+?>", "<a><b>")  # ['<a>', '<b>']

Practice Problems

1. Extract dates with lookahead
2. Match passwords using lookbehind
3. Parse log entries with named groups
4. Replace using backreferences
5. Validate complex patterns