What is a regular expression?
A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. It is used to match, find, and manipulate text. Regular expressions are supported in virtually every programming language — JavaScript, Python, Java, Ruby, Go, and many others — making them an indispensable tool for text processing. A regex can be as simple as a literal string or as complex as a pattern that validates email addresses or extracts structured data from logs.
Common regex patterns
Some frequently used patterns include: \d+ to match one or more digits, \b\w+\b to match whole words, ^ and $ to anchor to the start and end of a line, [a-z] for character classes, and (foo|bar) for alternation. The g flag enables global matching (find all occurrences), i makes the match case-insensitive, and m makes ^ and $ match line boundaries.
How this tool works
Enter your regular expression in the pattern field and any flags (such as g, i, or m) in the flags field. Paste or type your test string, then click Test Regex. The tool compiles the expression using JavaScript's built-in RegExp constructor and runs it against your input, listing all matches along with their positions in the string. If the expression is invalid, a clear error message is shown.
Use cases
Regex testers are used to validate input formats (email, phone numbers, dates), extract data from log files, find and replace text in editors, parse structured text like CSV or HTML, and write test cases for string processing functions. Testing a regex interactively before embedding it in code prevents subtle bugs and saves debugging time.