Question 1

What regex engine is used?

Accepted Answer

The tool uses the JavaScript native RegExp engine, compatible with modern regex features including lookaheads, lookbehinds, named capture groups, and Unicode property escapes. Patterns tested here work directly in Node.js and browser JavaScript.

Question 2

What flags are supported?

Accepted Answer

Global (g) to find all matches, case-insensitive (i), multiline (m) to make ^ and $ match line boundaries, dotAll (s) to make . match newlines, and unicode (u) for full Unicode support.

Question 3

How do I match a newline?

Accepted Answer

Use 
 to match a newline character in the pattern. If you want . to match newlines as well as other characters, enable the dotAll (s) flag.

Question 4

What are capture groups?

Accepted Answer

Capture groups are portions of a regex pattern enclosed in parentheses. They capture the matched text for each group separately. For example, (\d{4})-(\d{2})-(\d{2}) on "2023-11-14" captures "2023", "11", and "14" as three groups.

Question 5

What is the difference between a greedy and lazy quantifier?

Accepted Answer

Greedy quantifiers (like +, *) match as much as possible. Lazy/non-greedy quantifiers (like +?, *?) match as little as possible. For example, .+? stops at the first match, while .+ continues to the end of the line.

Question 6

Can I test patterns with Unicode characters?

Accepted Answer

Yes. Enable the unicode (u) flag to use Unicode property escapes like \p{Letter} and correctly handle surrogate pairs and characters outside the Basic Multilingual Plane.

Question 7

Is my data sent to a server?

Accepted Answer

No. All regex matching is performed locally in your browser using the native JavaScript RegExp engine. Your patterns and test strings never leave your device.

Question 8

Can I use this regex in other programming languages?

Accepted Answer

The JavaScript RegExp flavor is similar to but not identical to Python, Java, or PCRE regex. Lookaheads, basic quantifiers, and character classes are broadly compatible. Language-specific features (like Python's (?P<name>) syntax) will need adjustment.

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