Regex Tester

Regular expressions — regex — are patterns used to find and validate pieces of text: an email, a postal code, a sequence of digits, a specific word. Instead of describing what you're looking for in words, you describe it with a pattern of symbols.

In this tool you type the expression, check the flags you want and paste some test text. The matches found are highlighted inside the text, in real time, and the tool shows how many there are. If the expression is malformed, it warns you with the error message instead of just silently failing.

The flags adjust the behavior: "i" ignores case, "m" makes the start and end apply per line, "s" makes the dot match line breaks too. The "g" flag, which finds all occurrences, is applied automatically so the highlight shows every result, not just the first.

Developers use regex to validate form fields, extract data from text, run search-and-replace in bulk and filter logs. The problem is that a complex expression is easy to get wrong — and testing it straight in code, attempt after attempt, is slow.

A tester solves that: you tweak the pattern and immediately see what it captures, without leaving to run the whole program. It's ideal for building an expression step by step, understanding why a pattern isn't matching what it should, or learning regex by experimenting. It serves beginners and people who just want to double-check an expression before pasting it into a project.

A classic mistake is forgetting to escape special characters. Symbols like the dot, parentheses, brackets and backslash have their own meaning in regex. To match a literal dot, for example, you have to write it as "." — otherwise it matches any character.

Another slip is the "greedy" pattern. By default, quantifiers like "+" and "*" grab the longest possible stretch, which can capture far more than you intended. Understanding the difference between greedy and "lazy" matching (with "?") avoids surprising results.

There's also the flag confusion. Without the global flag, a search finds only the first occurrence; without the "i" flag, the search is case-sensitive. Many people think the expression is wrong when, in reality, the right flag was missing.