A regex tester is the essential tool for any developer working with regular expressions. Whether you're a beginner or an expert, knowing how to use it correctly saves considerable time during development and debugging.
An online regex tester consists of three main areas: the pattern field (the regular expression), the test text field and the results area. You enter your regex in the first field, paste the text to analyze in the second, and the tool displays in real time the highlighted matches, capturing groups and their content. Most testers also allow you to choose flags (g, i, m, s) that modify the behavior of the search.
📊 Reference table
| Flag | Name | Effect |
|---|---|---|
| g | Global | Finds all occurrences (not just the first one) |
| i | Case-insensitive | "abc" also matches "ABC" and "Abc" |
| m | Multi-line | ^ and $ match each line start/end |
| s | Dotall | The dot . also matches newlines |
| u | Unicode | Enables full Unicode support |
💡 Practical examples
Step 1: enter a simple pattern
In the Pattern field, type \d+ to find numbers. In the text, enter "I have 3 cats and 12 fish". The tool highlights 3 and 12.
Step 2: use capturing groups
Pattern: (\w+)@(\w+\.\w+) on "[email protected]" — Group 1: "user", Group 2: "example.com". Very useful for extracting sub-parts.
Step 3: debug a complex regex
If your regex doesn't match, check: (1) escaped characters (\. vs .), (2) quantifiers (* vs + vs ?), (3) the global flag g if you expect multiple results.