Regex Tester Online
Test and debug regular expressions in real-time. Highlights match patterns, captures match groups, and runs entirely in your browser.
Regular expressions (regex) are a core component of text processing in modern programming languages. Whether you are validating user forms, searching database tables, parsing log entries, or rewriting URL pathways, regex patterns provide a flexible and powerful way to match and capture substrings. Understanding regex syntax and flags allows developers to replace complex string parsing logic with simple, single-line matching declarations.
Core Syntax and Character Classes
A regular expression is composed of literal characters and special metacharacters that define matching logic. Common classes include:
| Symbol | Matches | Example |
|---|---|---|
. | Any character except newline | a.c matches abc, adc |
\d | Any numeric digit (0-9) | \d3 matches 123, 456 |
\w | Any word character (alphanumeric + underscore) | \w+ matches user_name |
\s | Any whitespace character (space, tab, newline) | \s+ matches multiple spaces |
^ / $ | Start / End of string (or line in multiline mode) | ^auth matches strings starting with auth |
* / + / ? | Quantifiers: 0 or more / 1 or more / 0 or 1 | a+ matches a, aa, aaa |
Capturing Data with Groups
Beyond checking if a pattern matches a string, regex is frequently used to extract subsets of data. Wrapping a portion of your pattern in parentheses (pattern) creates a capture group. The regex engine stores the matched value for each group, making it accessible as an array. For example, if you match a date string like 2026-10-12 with the pattern (\\d4)-(\\d2)-(\\d2), the engine captures group 1 as 2026, group 2 as 10, and group 3 as 12. This allows for quick, structured text parsing.
frequently asked questions
A regular expression is a sequence of characters that forms a search pattern. It is used to match, search, and manipulate text in software applications, such as validating emails, replacing strings, or parsing logs.
"g" (global) finds all matches in the text instead of stopping after the first one; "i" (case-insensitive) ignores letter casing; "m" (multiline) causes the start (^) and end ($) anchors to match the start and end of individual lines; and "s" (dotAll) allows the dot (.) character to match newline characters.
Capture groups are defined by wrapping a part of the regex pattern in parentheses (e.g. (\d+)). When a match is found, the parser extracts the substring that matches the group, making it easy to capture specific fields from text.
Yes, your text and expressions are processed completely inside your browser using the local JavaScript engine. No data is sent to external servers.
This can happen if you write an inefficient pattern that causes catastrophic backtracking, where the regex engine takes exponential time to evaluate combinations. To prevent freezes, test with simple patterns first.