Regular Expression Tester — Test & Debug Regex Patterns Online
Understanding the Basics
Regular expressions (regex or regexp) are a formal language for describing string patterns, first introduced by mathematician Stephen Kleene in the 1950s and subsequently incorporated into virtually every programming language and text-processing tool. A regex pattern is a sequence of characters where some have literal meaning (the character a matches only "a") and others are metacharacters with special pattern-matching semantics: the dot (.) matches any character, the asterisk (*) means "zero or more of the preceding element", the plus (+) means "one or more", the question mark (?) means "zero or one", and character classes like [a-z] match any lowercase letter. Grouping with parentheses creates capturing groups — subpatterns whose matched text is extracted separately from the full match. Alternation with | allows matching one of several alternatives. Anchors like ^ (start of string) and $ (end of string) constrain where matches can occur. Lookaheads (?=...) and lookbehinds (?<=...) assert context without including it in the match. Regex is used for input validation (email, phone, URL formats), text extraction from logs and documents, find-and-replace in editors and build tools, URL routing in web frameworks, and data parsing in ETL pipelines. Our tester uses JavaScript's native RegExp engine with real-time evaluation and group capture display.
How to Use the Tool
- Enter your regular expression pattern in the Pattern field (without surrounding slashes).
- Select regex flags: g (global — find all matches, not just first), i (case-insensitive), m (multiline — ^ and $ match line boundaries), and s (dotAll — dot matches newlines).
- Paste your test string into the text editor below.
- All matches are highlighted directly in the text and listed in the Matches panel with their index offsets and captured groups.
- Adjust the pattern and see matches update in real time as you type.
Key Features & Specifications
- Real-time regex evaluation with millisecond latency as you type the pattern
- All matches highlighted visually in the test text with distinct colors
- Capture group display: each group's matched text is shown alongside the full match
- Named capture groups (?<name>...) are shown with their names in the results panel
- Flag toggles for g, i, m, s, u, and y modes
Frequently Asked Questions (FAQ)
- Q:What regex engine does this tool use?
- The tool uses JavaScript's native ECMAScript RegExp engine. This covers almost all standard regex features, but differs from PCRE (used in PHP, Ruby, and Python's re module) in a few areas: JavaScript does not support lookbehind in older engines, recursive patterns, or POSIX character classes. Patterns must be valid ECMAScript regex syntax.
- Q:Why is my pattern matching more than I expect?
- Without anchors, a pattern can match anywhere in the string. Add ^ to require the match to start at the beginning and $ to require it to end at the string's end. Without the g flag, only the first match is returned even if many exist.
- Q:What is a capture group and how do I use it?
- Parentheses create a capturing group: the text matched by the sub-pattern inside them is captured and accessible separately. For example, (\d{4})-(\d{2})-(\d{2}) applied to "2024-06-20" captures three groups: "2024", "06", and "20". Named groups use (?<name>...) syntax.
- Q:How do I match a literal dot, asterisk, or plus sign?
- Metacharacters must be escaped with a backslash to match literally. To match a literal dot, use \. To match a literal asterisk, use \*. To match a backslash itself, use \\.