regex-tester

name	regex-tester
description	Create, test, and debug regular expressions. Use when user asks to write a regex, test a pattern against text, or debug why a regex isn't matching.
version	1.0.0
author	deskwand
license	MIT
metadata	{"tags":["Regex","Regular Expressions","Pattern","Matching","Text"]}
allowed-tools	read,shell,grep

Regex Tester

Create and test regular expressions interactively.

When to use

• User asks to "write a regex for..."
• User has a regex that isn't working as expected
• User wants to extract specific patterns from text
• User asks to validate email/phone/URL formats

---

Quick Test (Shell)

# Test if regex matches
echo "test string" | grep -oP 'pattern'

# Extract matches
echo "abc123def456" | grep -oP '\d+'
# Output:
# 123
# 456

# Count matches
echo "abc123def456ghi789" | grep -oP '\d+' | wc -l

# Show matching lines with context
grep -nP 'pattern' file.txt

---

Common Patterns

Validation Patterns

Pattern	Regex	Notes
Email	^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$	Simplified; RFC 5322 is more complex
URL	https?://[^\s/$.?#].[^\s]*	Basic; consider urllib.parse for robust parsing
IPv4	^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$	Does not validate ranges (0-255)
Date (YYYY-MM-DD)	^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$	Does not validate month-day combos
Phone (Chinese)	^1[3-9]\d{9}$	11-digit mobile
SemVer	^\d+\.\d+\.\d+(-[a-zA-Z0-9.]+)?(\+[a-zA-Z0-9.]+)?$

Extraction Patterns

Pattern	Regex
Numbers	\d+ (integers), \d+\.\d+ (decimals)
Words	\b\w+\b
Quoted strings	"([^"]*)" or '([^']*)'
HTML tags	<(\w+)[^>]*>(.*?)</\1>
Markdown links	\[([^\]]+)\]\(([^)]+)\)
Hex color	#([0-9a-fA-F]{6}|[0-9a-fA-F]{3})

---

Python Test Script

For complex regex debugging:

python3 -c "
import re

pattern = r'<your regex>'
tests = [
    ('should match', True),
    ('should not', False),
]

for text, expected in tests:
    result = bool(re.search(pattern, text))
    status = '✅' if result == expected else '❌'
    print(f'{status} \"{text}\" -> {result} (expected {expected})')
"

Regex Syntax Quick Reference

Element	Meaning
.	Any character except newline
*	0 or more
+	1 or more
?	0 or 1
{n}	Exactly n
{n,}	n or more
{n,m}	n to m
^	Start of string/line
$	End of string/line
\b	Word boundary
\d	Digit
\w	Word character
\s	Whitespace
[abc]	Character class
[^abc]	Negated class
(a|b)	Alternation
(...)	Capturing group
(?:...)	Non-capturing group
(?=...)	Lookahead
(?<=...)	Lookbehind

Flags

Flag	Python	grep -P
Case-insensitive	re.IGNORECASE / (?i)	grep -iP
Multiline (^/$ match line boundaries)	re.MULTILINE / (?m)	N/A
Dot matches newline	re.DOTALL / (?s)	N/A

Debugging Tips

1. Too greedy: Use .*? instead of .*
2. No match: Check if you need ^/$ anchors or word boundaries \b
3. Unexpected matches: Make the pattern more specific; avoid overly broad .*
4. Performance: Avoid nested quantifiers like (.*)* — catastrophic backtracking risk
5. Test edge cases: empty string, max length input, special characters, unicode

Constraints

• For production validation of emails/URLs, use dedicated parsers, not regex alone
• Never use user-provided regex directly without validation (ReDoS risk)
• Test with grep first, then with the target language's regex engine (dialects differ slightly)