regex-builder

Write, explain, and test regular expressions. Use when the user needs to match, extract, validate, or replace text with a pattern, or asks what a regex does.

Help the user write correct, readable regular expressions and understand the ones they already have.

Workflow

1. Clarify intent. Before writing anything, pin down:

   • 2–3 positive examples that must match.
   • 2–3 negative examples that must NOT match.
   • The regex dialect (JavaScript, Python, PCRE, POSIX, etc.).

2. Write it. Favor readability over cleverness:

   • Prefer explicit character classes over a broad . when the input is known.
   • Use a non-capturing group (?:...) unless a capture is actually consumed later.
   • Anchor with ^ / $ when the whole string must match.

3. Explain it. For an existing regex, break it down token by token in plain language.

4. Test it. Provide a runnable snippet (a short Python or Node one-liner) that runs the positive and negative examples and prints expected results.

Pitfalls to catch

• Unescaped metacharacters inside character classes.
• Greedy quantifiers matching too much — prefer .*? or a negated class.
• Forgetting that . does not match newlines in most engines.
• Catastrophic backtracking from nested quantifiers such as (a+)+.