Mastering regular expressions can quickly become a non-trivial task. While good text editors like e.g. Kate, KWrite or TextPad have good support for regular expressions, it can be difficult to construct complex regexps, and even more so to find out why they don’t work as intended if they fail.
I have found two resources which help with that: first, the Javascript Regular Expression Validator from net shift media, which takes a regular expression from you and provides two input fields for tests which can be matched against the expression – instantly, no need to press any buttons and wait for an answer. This is very handy to find out if there was a typo in your logic somewhere.
If you need more help in debugging your expressions, then Dr. Edmund Weitz’s RegEx Coach might be able to help. This Windows program (there’s an outdated, discountinued Linux version, too) will take you through the evaluation of your regular expression, step by step:

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