As you type your regular expression the background of the box will indicate whether your regex is valid. Red means your regular expression is currently invalid. This will happen when writing matching groups and classes are opened without close, forward slashes are unescaped, etc. Yellow indicates a valid regular expression that does not match any of your input text. Green indicates both a valid expression and a match in your input.
You can click on the main text area to edit the input test your regular expression will match against. General regex matches will be highlighted in yellow. If your regular expression uses matching groups the groups will be highlighted in other colors, too. Finally, regexp matches will be printed in the match table below your input.
To the right of the tool is a cheat sheet for quick reference on the syntax of regular expressions. It is organized by the primary features of the perl regex language.
Here are a few quick regex examples to get you started:
123-456-7890 1234567890 (123)456-7890
Hello World this does not Match!
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