Regex Tester
Test and debug regular expressions in real-time with match highlighting, capture group details, and common pattern presets.
What is a Regular Expression?
A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regex is a powerful tool used in virtually every programming language — JavaScript, Python, Java, PHP, Ruby, Go, and many others — for pattern matching within strings. Whether you're validating form inputs, parsing log files, extracting data from documents, or performing complex find-and-replace operations, regular expressions make the job dramatically simpler.
How Regex Works
At its core, a regex pattern describes a set of strings. The regex engine takes your pattern and attempts to match it against an input string character by character. Special characters like . (any character), * (zero or more), + (one or more), ? (optional), [] (character classes), () (capture groups), and anchors like ^ and $ give you incredible flexibility to define precisely what you're looking for.
Common Regex Patterns
- Email validation:
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}— matches most standard email addresses. - URL matching:
https?:\/\/[^\s/$.?#].[^\s]*— captures HTTP and HTTPS URLs from text. - Phone numbers:
\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}— matches US phone numbers in various formats. - IPv4 addresses:
\b(?:\d{1,3}\.){3}\d{1,3}\b— finds IP addresses in logs and config files. - Dates (YYYY-MM-DD):
\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])— matches ISO 8601 date formats.
Understanding Regex Flags
Flags modify how the regex engine interprets your pattern. The g (global) flag finds all matches rather than stopping at the first one. The i (case-insensitive) flag makes matching ignore letter case — so /hello/i matches "Hello", "HELLO", and "hello". The m (multiline) flag changes how ^ and $ work, making them match the start and end of each line rather than the whole string. Combining flags like gim gives you maximum flexibility for complex text processing.
Capture Groups and Backreferences
Parentheses in regex do double duty: they group parts of the pattern together and they capture the matched text for later use. For example, (\d{3})-(\d{4}) matches a phone prefix and line number, capturing each part separately. In replacement operations, you can reference these groups with $1, $2, etc. Named groups like (?<year>\d{4}) add even more clarity. Backreferences like \1 let you match the same text that was captured earlier — useful for finding repeated words or matching HTML tags.
Why Use Our Regex Tester?
Writing regex by hand is notoriously tricky — what looks correct often contains subtle bugs. Our tester gives you instant visual feedback as you type, highlighting every match in your test string. You can see match counts, inspect individual capture groups, and toggle flags to understand exactly how your pattern behaves. The preset library gives you battle-tested starting points for common validation tasks. Best of all, everything runs locally in your browser — your data never leaves your machine.
Whether you're a beginner learning regex for the first time or a seasoned developer debugging a complex pattern, our regex tester helps you get it right faster. Bookmark this page and make it part of your daily development toolkit.