Question 1

What regex flavor does this tester use?

Accepted Answer

The tester uses JavaScript's built-in RegExp engine. JavaScript regex is based on the ECMAScript standard and supports features like lookaheads, lookbehinds (ES2018+), named capture groups, and Unicode property escapes. It is compatible with Node.js, browsers, and most JavaScript environments.

Question 2

What does the "g" flag do in a regular expression?

Accepted Answer

The "g" (global) flag makes the regex engine find all matches in the string rather than stopping at the first one. Without "g", only the first match is returned. For most use cases in search-and-replace operations, you want the "g" flag enabled.

Question 3

How do I match a literal dot or special character?

Accepted Answer

In regex, dots (.) and other special characters have special meanings. To match a literal dot, escape it with a backslash: \.. For example, the pattern \d+\.\d+ matches numbers like "3.14". Other characters needing escaping include: ^ $ * + ? { } [ ] \ | ( )

Question 4

What is a capture group and how do I use them?

Accepted Answer

A capture group is a portion of the regex wrapped in parentheses that captures the matched substring for later use. For example, (\d{4})-(\d{2})-(\d{2}) applied to "2024-01-15" captures "2024", "01", and "15" as groups 1, 2, and 3. Named groups use (?...) syntax: (?\d{4}).

Question 5

What is the difference between greedy and lazy quantifiers?

Accepted Answer

Greedy quantifiers (*, +, {n,}) match as much as possible. Lazy quantifiers (*?, +?, {n,}?) match as little as possible. For example, on "bold", the greedy pattern <.*> matches the entire string, while the lazy pattern <.*?> matches just "".

Regex Tester