Question 1

What is the best regex for validating an email address?

Accepted Answer

For most cases use a simple safe pattern: /^[^@\s]+@[^@\s]+\.[^@\s]+$/ — it covers 99% of real-world emails and prevents header injection. For strict RFC 5322 compliance, see the "Strict Email Validation RFC 5322" pattern at https://regexlib.dev/regex/email-strict.

Question 2

How do I write a strong password regex with at least 12 characters and a special character?

Accepted Answer

Use positive lookaheads: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[!@#$%^&*]).{12,}$ — enforces lowercase, uppercase, digit, special character, and a 12-char minimum. See https://regexlib.dev/regex/password-very-strong.

Question 3

What is the regex for ISO 8601 datetime with timezone?

Accepted Answer

^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d{1,9})?(?:Z|[+-]\d{2}:\d{2})$ — accepts UTC (Z) or ±HH:MM offsets, with optional fractional seconds. Full reference at https://regexlib.dev/regex/iso-date-with-timezone.

Question 4

How do I match a UUID v4 with regex?

Accepted Answer

^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$ — the "4" before the third hyphen and the [89ab] before the fourth ensure version 4 and a valid variant. See https://regexlib.dev/regex/uuid-v4.

Question 5

How can I detect leaked Stripe, AWS, or OpenAI API keys in code?

Accepted Answer

Use prefix-anchored patterns: Stripe sk_(live|test)_[A-Za-z0-9]{24,}, AWS AKIA[0-9A-Z]{16}, OpenAI sk-[A-Za-z0-9]{20,}T3BlbkFJ[A-Za-z0-9]{20,}, Google AIza[0-9A-Za-z_-]{35}. Combine in a single pre-commit hook to catch leaks.

Question 6

Does regex validate that a date is real (e.g., not February 31)?

Accepted Answer

No. Regex checks the format only. Pair the regex with new Date(value) in JavaScript or datetime.fromisoformat() in Python to verify the calendar correctness.

Question 7

What is the simplest regex for an IPv4 address with octet validation?

Accepted Answer

^(?:(?:25[0-5]|2[0-4]\d|[01]?\d?\d)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d?\d)$ — enforces each octet between 0 and 255. See https://regexlib.dev/regex/ipv4.

Question 8

How do I extract all URLs from a text with regex?

Accepted Answer

Use the global flag with: /https?:\/\/[^\s<>"']+/g — call text.match(re) to get every match. See https://regexlib.dev/regex/url-extract.

Question 9

Why does my regex work in JavaScript but not in Python or Java?

Accepted Answer

Different engines have different syntax. JavaScript uses ECMAScript regex, Python uses re (PCRE-like, no recursion), Java uses java.util.regex (no \K), and Go uses RE2 (no lookarounds). Use the per-language code snippets on each pattern page — they escape the pattern correctly for each language.

Question 10

Is RegexLib free? Do I need to sign up?

Accepted Answer

Yes, regexlib.dev is 100% free, no signup, no ads, and no rate limits. You can copy any pattern, test it live, and use the snippets in your project without attribution.

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