Question 1

What regex flavour does this tool use?

Accepted Answer

The tester uses JavaScript's built-in RegExp engine (ECMAScript 2022+), which supports named capturing groups, lookbehind assertions, Unicode property escapes (\p{L}), the dotAll (s) flag, and the sticky (y) flag. It does not support PCRE-only features like \K or recursive patterns.

Question 2

What does the g (global) flag do?

Accepted Answer

Without the g flag, a regex stops after the first match. With g, it finds all non-overlapping matches in the string. Most operations in this tester (match count, highlighting, substitution) implicitly behave as if g is set regardless of the flag selection.

Question 3

How do I match a literal dot or other special character?

Accepted Answer

Escape it with a backslash: \. matches a literal dot (without the backslash, . matches any character except newline). Other characters that need escaping: \ ^ $ . | ? * + ( ) [ ] { }.

Question 4

What is a capturing group and how do I use it?

Accepted Answer

Parentheses create a capturing group: (\d{4}) captures four digits. In the substitution field, reference it as $1 (first group), $2 (second), etc. Use (?:...) for a non-capturing group when you need grouping for repetition but not the captured value.

Question 5

What is the difference between \d and [0-9]?

Accepted Answer

In JavaScript without the u or v flag, they are equivalent for ASCII digits. With the Unicode (u) flag and Unicode property escapes, \d still matches only ASCII 0–9, whereas \p{Decimal_Number} matches decimal digits from all scripts. For most use cases they are interchangeable.

Regex Tester

How to use the Regex Tester

What is a regular expression?

Regex flags explained

Capture groups and substitution

Commonly used regex patterns