regex-mastery

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Regex Mastery

Regular expressions are a compact language for describing text patterns, built into virtually every programming language and text processing tool. They power input validation, log parsing, data extraction, search-and-replace, and tokenization. Used well, a single regex can replace dozens of lines of string manipulation code. Used poorly, they become unreadable traps and can grind a server to a halt via catastrophic backtracking.

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When to use this skill

Trigger this skill when the user:

• Asks to write or explain a regular expression
• Wants to validate input format (email, URL, phone number, date, credit card)
• Needs to extract data from structured or semi-structured text (logs, CSV, HTML)
• Uses regex terminology: lookahead, lookbehind, named group, capture group, backreference
• Wants to debug a pattern that isn't matching as expected
• Asks about regex flags (i, g, m, s, u, x)
• Needs to replace text using capture groups or back-references

Do NOT trigger this skill for:

• Full HTML/XML parsing (use a proper parser like DOMParser or BeautifulSoup instead)
• Complex natural language processing where ML models are a better fit

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Key principles

1. Readability over cleverness - A regex that nobody can maintain is worse than a slightly longer explicit approach. Break complex patterns into commented steps or use the verbose (x) flag where supported. A named group costs nothing but pays dividends every time someone reads the pattern.
2. Use named capture groups - (?<year>\d{4}) is self-documenting and immune to positional breakage when the pattern changes. Always prefer named groups over numbered groups for any regex that will be read or maintained by humans.
3. Test edge cases relentlessly - Empty string, Unicode characters, very long input, malformed-but-close input (e.g., foo@bar for email), and adversarial input designed to trigger backtracking. A regex that passes your happy path but fails on a Unicode em-dash will cause production incidents.
4. Avoid catastrophic backtracking - Nested quantifiers ((a+)+) and overlapping alternatives ((a|ab)+) cause exponential backtracking on non-matching input. Use atomic groups or possessive quantifiers where available, or restructure alternation so choices are mutually exclusive.
5. Use the right tool - Regex is not always the answer. Parsing emails to RFC 5321 compliance requires a full parser. Parsing JSON, HTML, or XML requires a DOM/SAX parser. If a regex exceeds ~80 characters or requires >2 levels of nesting, pause and ask whether a small state machine or parser would be clearer.