Question 1

What is a regular expression (regex)?

Accepted Answer

A regex is a pattern-matching language used to search, validate, and extract text. Common use cases: validating email addresses, parsing log files, extracting URLs from HTML, replacing text patterns. Toolhub's tester uses the JavaScript regex flavor (similar but not identical to Python, Java, and PCRE).

Question 2

What regex flavor does Toolhub's tester use?

Accepted Answer

JavaScript's RegExp object, which supports ECMAScript regex syntax. This is similar to PCRE but lacks lookbehind assertions (until ES2018), named capture groups use a different syntax (?<name>...), and some Unicode features differ. For PCRE-compatible testing, use a dedicated tool like regex101.com.

Question 3

How do I capture groups in regex?

Accepted Answer

Wrap the part you want to capture in parentheses: '(\d{4})-(\d{2})-(\d{2})' captures year, month, and day separately. Use non-capturing groups '(?:...)' when you need grouping but don't want to capture. Named groups in JavaScript use '(?<name>...)'. Toolhub shows all capture groups in the test results.

Question 4

Can I use regex to validate email addresses?

Accepted Answer

Yes, but be careful — email regex is notoriously complex. A simple version: /^[^@]+@[^@]+\.[^@]+$/. A more permissive RFC 5322-compliant regex is hundreds of characters long. For production, prefer sending a verification email rather than relying on regex validation alone.

Regex Tester — Test Regular Expressions

Regex Tester

How to use this tool

Frequently Asked Questions

Regex Tester

How to use this tool

Frequently Asked Questions

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