Regex Puzzles — Test Your Mastery

Five puzzles, increasing difficulty

If you can solve all five, you've internalized this quest. Try each in your head first; check the answer at the bottom only after you have a candidate.

Puzzle 1: Match palindromes of length 3-5

Words that read the same forwards and backwards: 'mom', 'eye', 'level', 'civic'. Use word boundaries.

Puzzle 2: Match 'fizz' but not when preceded by 'buzz'

Find 'fizz' in text, but skip occurrences where 'buzz' immediately precedes (with optional space).

Puzzle 3: Match strings with EXACTLY two vowels

'cat' (1), 'beat' (2), 'beats' (2), 'beautiful' (5). Only 'beat' and 'beats' should match.

Puzzle 4: Strip ANSI color codes

Terminal output often has codes like \x1b[31m or \x1b[1;32m. Remove them all without removing other content.

Puzzle 5: Find duplicate adjacent words

Detect things like 'the the cat' or 'sat sat'. Return the duplicated word.

Spoilers (after you've tried)

1. \b(\w)(\w?)(\w)?\3\2?\1\b — capturing pairs around the center, using backreferences and optional matching.

2. (?<!buzz\s?)fizz — negative lookbehind. (Variable-width lookbehind needed; Python regex module or fixed-width pad.)

3. ^[^aeiou]*[aeiou][^aeiou]*[aeiou][^aeiou]*$ — non-vowels, vowel, non-vowels, vowel, non-vowels.

4. \x1b\[[\d;]*m — escape, bracket, digits and semicolons, m.

5. \b(\w+)\s+\1\b — backreference to detect repetition.

The graduation

If you understood all five — including the trade-offs (Puzzle 2's lookbehind variability, Puzzle 1's center-character handling) — you've graduated. The rest is practice. Keep regex101 bookmarked, treat lookaround as a power tool not a default, and remember the Golden Rule: smallest tool that solves the shape problem.

Code

Puzzle solutions·python

import re

# Puzzle 1: 3-5 letter palindromes
PALINDROME = re.compile(r'\b(\w)(\w?)(\w)?\3?\2?\1\b')
for m in PALINDROME.finditer('mom eye level civic noon racecar test'):
    word = m.group()
    if word == word[::-1] and 3 <= len(word) <= 5:
        print(word)
# mom
# eye
# level
# civic
# noon

# Puzzle 2: 'fizz' not preceded by 'buzz'
# Python re needs fixed-width lookbehind
re.findall(r'(?<!buzz )fizz', 'buzz fizz solo fizz buzzfizz')
# ['fizz', 'fizz']  — middle and end

# Puzzle 3: exactly two vowels
TWO_VOWELS = re.compile(r'^[^aeiou]*[aeiou][^aeiou]*[aeiou][^aeiou]*$')
for word in ['cat', 'beat', 'beats', 'beautiful']:
    if TWO_VOWELS.fullmatch(word):
        print(word)
# beat
# beats

# Puzzle 4: strip ANSI codes
ANSI = re.compile(r'\x1b\[[\d;]*m')
clean = ANSI.sub('', '\x1b[31mhello\x1b[0m \x1b[1;32mworld\x1b[0m')
print(clean)  # 'hello world'

# Puzzle 5: duplicate adjacent words
DUPS = re.compile(r'\b(\w+)\s+\1\b')
for m in DUPS.finditer('the the cat sat sat by'):
    print(m.group(1))
# the
# sat