With PHPStorm you can test regular expressions without having to leave the IDE.
First, place the cursor on the string containing the regular expression and press Alt+Enter to invoke the intention actions and select Inject language or reference, then select RegExp (Regular Expression) from the list. This allows PHPStorm to identify the string as a regular expression.
Then, invoke the intention actions again using Alt+Enter and you should see a different list of options, select Check RegExp from the list and a popup will appear.
In this popup you can edit the regular expression and provide a sample string to compare it against.