Regex Builder

The Problem

Regular expressions are powerful but notoriously hard to write and even harder to read. A single misplaced quantifier can match too much or nothing at all. Developers spend 30 minutes on regex101 tweaking patterns when AI can build and explain the correct regex in seconds.

The Prompt

You are a regex expert. Build a regular expression for the following requirement:

WHAT TO MATCH:
[describe in plain English what you want to capture — e.g., "email addresses with subdomains" or "dates in DD/MM/YYYY format"]

LANGUAGE/ENGINE: [e.g., JavaScript, Python, PCRE, Go]

EDGE CASES TO HANDLE:
[list any special cases — e.g., "should not match dates with month > 12"]

Provide:
1. **Regex**: The complete pattern
2. **Breakdown**: Explain each part of the regex on its own line
3. **Test Cases**: 5 strings that should match and 5 that should not
4. **Flags**: Which flags to use and why (g, i, m, etc.)
5. **Code Snippet**: Ready-to-use implementation in the target language

Example Output

Regex: /^(?<year>\d{4})-(?<month>0[1-9]|1[0-2])-(?<day>0[1-9]|[12]\d|3[01])$/

Breakdown:
^                   → Start of string
(?<year>\d{4})      → Named group "year": exactly 4 digits
-                   → Literal hyphen separator
(?<month>0[1-9]|1[0-2]) → Named group "month": 01-12
-                   → Literal hyphen separator
(?<day>0[1-9]|[12]\d|3[01]) → Named group "day": 01-31
$                   → End of string

Should match:    2026-04-15, 2000-01-01, 1999-12-31, 2026-02-28, 2030-06-30
Should NOT match: 2026-13-01, 2026-00-15, 26-04-15, 2026/04/15, 2026-04-32

When to Use

Use this skill whenever you need to validate input formats, extract structured data from text, or parse log files. It is especially useful when you need to handle edge cases that simple string matching cannot cover, or when reading someone else’s regex that looks like hieroglyphics.

Pro Tips

• Always specify the regex engine — JavaScript regex differs from Python’s re module, which differs from PCRE. Named groups, lookbehind support, and Unicode handling vary significantly.
• Ask for the negative cases first — telling the AI what should NOT match is often more important than what should, because false positives cause worse bugs than false negatives.
• Request a commented version — for complex patterns, ask the AI to output the regex with inline comments using the x flag (verbose mode) in Python or PCRE.