Regex Tester
Test regular expressions and view match results.
Overview
A tool for testing regular expressions in real time and viewing match results. Useful for pattern writing, debugging, and learning.
Formula
Regex engine operation: Matches patterns using NFA (nondeterministic finite automaton) with backtracking or DFA (deterministic finite automaton). At each position in the input string, the engine attempts to match from the start of the pattern, interpreting metacharacters (., *, +, ?, [], (), |, ^, $, \d, \w, etc.). Flags (g: global, i: case-insensitive, m: multiline, s: dotAll) modify matching behavior.
How to Use
- 1Enter the regex pattern.
- 2Enter the test string.
- 3Set flags (g, i, m, s, etc.).
- 4Matching results are highlighted in the text.
- 5Review capture group information.
Tips
- ✔Understand the difference between greedy matching (*) and lazy matching (*?).
- ✔For complex patterns like email validation, refer to well-known regex patterns.
- ✔\d means [0-9], \w means [a-zA-Z0-9_], and \s means whitespace characters.
- ✔Lookaheads (?=...) and lookbehinds (?<=...) check conditions without consuming characters.
- ✔Named capture groups (?<name>...) improve readability for complex regex.
FAQ
Q. What is the difference between using the g flag and not using it?
Without the g (global) flag, only the first match is returned. With the g flag, all matches in the entire string are found. In JavaScript, use matchAll() to get all matches with their capture groups.
Q. When does regex performance become slow?
Nested quantifiers (e.g., (a+)+) or overlapping alternatives can cause 'catastrophic backtracking'. This can be resolved with atomic groups or possessive quantifiers.
Q. Do regex differ between programming languages?
The basic syntax is similar, but there are differences in lookbehind support, Unicode handling, and available flags. It is best to check the regex engine documentation for each language (JavaScript, Python, Java, Go, etc.).
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