Regex Tester

What Are Regular Expressions?

Regular expressions (regex or regexp) are patterns used to match character combinations in strings. They are one of the most powerful tools in a developer's toolkit, supported by virtually every programming language, text editor, and command-line utility. A well-crafted regex can validate input, extract data, find and replace text, and parse complex string formats in a single expression.

When to Use Regex

Common use cases include validating email addresses, phone numbers, and URLs; extracting data from log files or HTML; searching code for patterns; parsing CSV or structured text; and implementing search-and-replace operations. Regex is especially valuable when the pattern you need to match varies — for example, finding all dates in a document regardless of format, or extracting every URL from a block of text.

Testing Patterns Before Deploying

Regular expressions can be tricky to get right on the first try. Subtle mistakes like forgetting to escape a special character, missing an edge case, or creating an overly greedy quantifier can lead to unexpected matches or missed results. This tool lets you test your patterns against sample text in real time, see exactly what matches and where, inspect capture groups, and iterate until your expression works perfectly — all before writing it into production code.

Understanding Flags

Regex flags modify how the pattern engine behaves. The global (g) flag finds all matches instead of stopping at the first one. Case insensitive (i) makes the pattern match regardless of letter case. Multiline (m) changes ^ and $ to match the start and end of each line rather than the entire string. DotAll (s) makes the . character match newline characters as well. This tester runs entirely in your browser using JavaScript's native RegExp engine — your data never leaves your device.