Regex Wizard

Regex Wizard

Generate, test, and explain regular expressions from plain English.

Requirements

• python3, grep -P, or node for testing regex (at least one)
• No API keys needed

Instructions

Generate regex from description

1. Clarify ambiguous requirements (multiline? global? case-insensitive?)
2. Produce the pattern with flavor noted (Python, JS, PCRE, Go)
3. Explain each component in plain English
4. Show 3+ example matches and 2+ non-matches

Explain existing regex

1. Break into tokens/groups with inline annotations
2. Explain in simple language (avoid jargon)
3. Show example strings that match and don't match
4. Note the assumed flavor and any flavor-specific features used

Test regex against strings

Run the pattern against provided test strings:

# Python (most portable)
python3 -c "
import re
pattern = r'YOUR_PATTERN'
tests = ['test1', 'test2', 'test3']
for t in tests:
    m = re.search(pattern, t)
    print(f'{t!r}: {\"✅ Match\" if m else \"❌ No match\"}' + (f' → groups: {m.groups()}' if m else ''))
"

# Grep (quick check)
echo "test string" | grep -P 'pattern'

Output format

**Pattern**: `^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`
**Flavor**: Python/PCRE
**Flags**: None

| Component | Meaning |
|-----------|---------|
| `^` | Start of string |
| `[a-zA-Z0-9._%+-]+` | One or more valid email chars |
| `@` | Literal @ |
| `[a-zA-Z0-9.-]+` | Domain name |
| `\.[a-zA-Z]{2,}$` | TLD (2+ letters) at end |

**Matches**: `user@example.com`, `a.b+c@test.co.uk`
**Non-matches**: `@example.com`, `user@`, `user@.com`

Common Pitfalls to Warn About

• Greedy vs lazy: .* vs .*? — explain when it matters
• Catastrophic backtracking: Nested quantifiers like (a+)+ — always flag these
• Unescaped dots: . matches any char, not just literal dots
• Anchors: Missing ^/$ can cause partial matches
• Unicode: \w behavior differs across flavors (Python 3 includes Unicode by default)

Edge Cases

• Email/URL validation: Warn that perfect regex for these is extremely complex. Suggest using libraries instead for production.
• Multi-line input: Remind about re.DOTALL / re.MULTILINE flags.
• Empty string matching: Patterns like .* match empty strings — clarify if that's intended.

Security

• Never use user-supplied regex directly in production without timeout/limits (ReDoS risk).
• When testing regex against user data, sanitize the test strings in output.