Regex Tester & Builder

A regex tester is an online tool that lets you type a regular expression and instantly see which parts of your test string match. It highlights matches in real time so you can debug and refine your pattern without running code.

In JavaScript you can use const matches = text.match(/your-pattern/g) or the newer [...text.matchAll(/pattern/g)]. This tool generates the exact JavaScript snippet for your regex with one click — just switch to the "Code Snippet" tab.

Capture groups let you extract parts of a match. Wrap part of your pattern in parentheses: (\w+)@(\w+\.\w+). The first group $1 captures the username and $2 captures the domain. Use named groups (?<name>pattern) for readability.

The "g" (global) flag tells the regex engine to find ALL matches in the string rather than stopping after the first. Without "g", most regex functions return only the first match.

Import the re module and use re.findall(r"pattern", text) for all matches, re.search(r"pattern", text) for the first match, or re.sub(r"pattern", "replacement", text) to replace. This tool generates the Python snippet automatically from your pattern.

A common email regex is: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}. You can load this instantly from the "Pattern Builder" panel under the Contact category. Note: fully RFC-5321 compliant email validation requires a much more complex pattern.

A precise IPv4 regex is: \b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b — this validates that each octet is 0-255. Find it in the Network category of the Pattern Builder.

Greedy quantifiers (*, +, {n,m}) match as much text as possible. Lazy quantifiers (*?, +?, {n,m}?) match as little as possible. For example, <.+> on "<b>bold</b>" matches the entire string greedily, while <.+?> matches just "<b>".