RegexPal: Web-Based Regex Testing Reinvented

Yes I know, there are many other JavaScript regex testers available. Why did I create yet another? RegexPal brings several new things to the table for such web-based apps, and in my (biased) opinion it's easier to use and more helpful towards learning regular expressions than the others currently available. Additionally, most other such tools are very slow for the kind of data I often work with. They might appear fast when displaying 10 matches, but what about 100, 1000, or 5000? Try generating 5,000 matches (which is easy to do with an any-character pattern such as a dot) in your favorite existing web-based tool and see if your browser ever recovers (doubtful). The same task takes RegexPal less than half a second, and what's more, results overlay the text while you're typing it.

At the moment, RegexPal is short on features, but here are the highlights:

  • Real-time regex syntax highlighting with backwards and forwards context awareness.
  • Lightning-fast match highlighting with alternating styles.
  • Inverted matches (match any text not matched by the regex). screenshot

I'm not sure when I'll add additional features, but there are lots of things I'm considering. If there is something you'd like to see, let me know.

A few things to be aware of:

  • The approach I've used for scrollable rich-text editing (which I haven't seen elsewhere) is fast but a bit buggy. Firefox 2 and IE7 have the least issues, but it more or less works in other browsers as well.
  • The syntax highlighting generally marks corner-case issues that create cross-browser inconsistencies as errors even if they are the result of browser bugs or missing behavior documentation in ECMA-262 v3.
  • There are different forms of line breaks cross-platform/browser. E.g., Firefox uses \n even on Windows where nearly all programs use \r\n. This can affect the results of certain regexes.

At least for me, RegexPal is lots of fun to play with and helps to make learning regular expressions easy through its instant feedback. I encourage you to just go play with it and discover its results on your own, but for the curious, I'll keep rambling…

Regex syntax parsing (needed for the syntax highlighting) is somewhat complex, due to the numerous backwards and forwards context awareness issues involved. Take, for example, the pattern \10. What does it mean?

  • Backreference 10, if not inside a character class and at least 10 capturing groups are opened before that point.
  • Backreference 1, followed by a literal "0", if not inside a character class and between 1 and 9 capturing groups are opened before that point.
  • Octal character index 10 (decimal 8), if inside a character class, or if no capturing groups are opened before that point.
  • The three literal characters "\", "1", and "0", if preceded by an unescaped "\" character.
  • An incomplete token in a couple other situations.

Another example is the "-" character. Outside a character class it's always a literal hyphen, but inside a character class…

  • It creates a range between tokens if:
    • There is a preceding and following token in the class, or it's preceded by a token and is the last character in an unclosed character class (caveats follow).
  • It's a literal character if:
    • It's the first or last character in the class.
    • It's preceded by an unescaped "\".
    • It follows a token which is the end index for a range.
    • It follows a hyphen which creates a range.
  • It's an error if:
    • It's creating a range between tokens in reverse character index order (e.g., z-a, @-!, \uFFFF-\b, or \127-\cB).
    • It would otherwise create a range, but it's followed or preceded by a token which represents more than one character index (e.g., \d). In fact, in some cases browsers take this to mean that the hyphen should be treated as a literal, but browser bugs cause it to be handled inconsistently so RegexPal flags it as a range error.

Here are a few more things which aren't errors but are flagged as such:

  • Empty, top-level alternation, except at the end of the pattern, where such an alternation is ignored when highlighting matches in order to create a less surprising experience while the user is in the middle of constructing the regex. Empty, top-level alternation is flagged as an error because it effectively truncates the regex at that point (since it will always match). If a zero-length, top-level alteration is really needed, there are other easy ways to do that more explicitly.
  • Lookaround quantifiers (e.g., the plus sign in (?!x)+). This would be an actual error with some regex libraries (e.g., PCRE), and although that's not the case in most web browsers, such constructs add no value. As a result, RegexPal flags such quantifiers as an error, since they are almost certainly a user mistake.
  • \c when not followed by A–Z, \x when not followed by two hex characters, and \u when not followed by four hex characters. Although these do not cause most browsers to throw errors, they are handled inconsistently cross-browser and are hence flagged as errors. They would almost certainly be a user mistake even if the cross-browser issues didn't exist.

Credit to from where the text of the short-and-sweet Quick Reference is based, and to RegexBuddy from JGsoft for inspiring many of RegexPal's features. The name RegexPal is, in part, a nod to RegexBuddy, but also selected because it contains both "regex" and "regexp." wink

54 thoughts on “RegexPal: Web-Based Regex Testing Reinvented”

  1. You know what’s funny? I came upon RegexPal today in my Programming RSS feed and before the link was open I came here to see what you had to say about it. It turns out you made it! Your intense passion for and knowledge of regexes is so profound that now whenever I think of regex, I think of this site. Well done, sir.

  2. Steve is a master of Regexery. Nevertheless, whenever I think of having too much time on my hands, I think of this site.

  3. I just found myself having to write a regex for work, and suddenly I remembered Ryan mentioning RegexPal to me a few weeks ago. I tried it out and it works like a charm. Nicely done, sir.

  4. Fantastic tool and I agree; much easier to use than any other I’ve found. One quick question – is it possible to tell it to search over the end of lines? I’ve been building a suite of regexes to parse c++ headers to retrieve the public interfaces. It would have been nice to be able to highlight properly the following matched comment:

    /* some multiline comment
    which my software is able to strip
    with the regex your site helped me build 🙂 */

    The regex your tool helped me build worked, but it would be even better still if there was a “Treat multiline input as a single string” option.

    One other enhancement that would be fantastic would be the ability to pipe one regex output into a second edit box and perform another regex on that. Add that and you’ve got a very powerful tool to build multi-level regexes …and I would be a very grateful little developer! 😉

  5. The problem with using visual tools to justify something capable of matching invisible items… what happens when you want to test the matching of invisible items with a visual tool (highlighting)?

  6. Hi, I also posted about the linebreak issue earlier. I recommend having a ‘show all characters’ or ‘show invisible character highlights’ option to remedy that.
    Also I noticed this ‘bug’ in the wordwrap behavior of textarea, you can see it here:

  7. A great addition would be a little notification of the milliseconds or microseconds it took to find all the matches within the haystack. The speed of different regex patterns is something I always want to test as well.

  8. I find your testers by google and it’s very useful!
    I’m French and I will give the URL at all of my friends!
    Thank you very much!

  9. Before anything else, a million thanks to Steve for this great tool for webdevs all over the world!

    I was trying out lookbehind (?<=) and found that it did not work even though that lookahead works normally. Anyone else having this problem?

    Furthermore, I’ve been wondering about the various colors used in the regexpal application. I know for certain that yellow highlight is for positive results, but I haven’t figured out what blue highlights do at the time being.

  10. Thanks for the suggestions, bug reports, and compliments, everyone.

    @k4emic, JavaScript doesn’t support lookbehind, so RegexPal correctly highlights your group as an error (red). As for the highlighting of match results, they simply alternate between yellow and blue. This allows you to easily determine where one match ends and the next begins, even when two matches are adjacent to each other.

  11. You have a great online tester there. I love that it not only highlights matches – but different sets of matches are different colors! Forget the stuff I was using – your site is now my new tester!

  12. Couldn’t get it to work, nothing in my test data would highlight (though my source regex would). I think it would be much more useful if on page load it was populated with an example.

  13. I LOVE YOUR REGEX TESTER. It has some highlighting issues in FF but works correctly with IE. I’ve only experienced the highlighting issue once though, the highlight was shifted. It worked fine in IE though. If I can find the regex and data ill be sure to post it for you! THANK AGAIN!

  14. Thank you for your great tool!!!
    I only would add a message indicating that yellow colour means a match with the expression.

  15. Hey man, I have a suggestion for this wonderful tool.

    What do you think of the groups highlight the text with regexp in the same colors!?


  16. Guess I am stupid . . . So, when I enter my pattern and then my test data what do I do next? I have no idea how to tell it “GO!”

  17. If you paste a regular expression in first and then paste test data in second it does nothing. You must modify the regular expression in order to get it to “GO.” Trivial enough once you realize that’s the trick but initially confusing.

Leave a Reply

Your email address will not be published. Required fields are marked *