I've used RegexBuddy several times and found it to be a really useful tool. Is there anything like it in the open source world, preferably something that is platform agnostic? (the fact that regexbuddy is windows only is a real downer)
The only thing I've been able to find is Kodos, which I've also used quite a lot, but it doesn't quite have the features of regexbuddy.
I commonly use: RegexLib.com the ads can be annoying, but at least it is online so I never have to install anything.
web-based reg-ex evaluator: http://www.rexv.org/
It's not as thorough as RegexBuddy, but it's platform agnostic, free, and works in 90% of the situations in which I want to use RegexBuddy.
(then again, I often just run RegexBuddy in wine if I'm on linux)
I suggest taking a look at The Regulator, it's probably as close to RB as you're going to get!
KDE has a nice one called KRegexpEditor that's pretty helpful.
There's also regexxer if you prefer GTK.
I use an Eclipse plug-in called Regex Tester. You can find information about it here. It has a live evaluation feature, which is really cool because you can see what parts of a sample string will match while you're still typing the regular expression. I find it to be extremely helpful.
Your best bet is to use 2 tools from Roy Osherove.. Regulator and Regulazy. Regulator is similar to RegEx Buddy, it allows you to test and validate your regular expressions but Regulazy takes a unique approach of allowing you type in the text you want to match and it generates the regex for you.
You can get both from: http://tools.osherove.com/
I've never used RegexBuddy, but if you want a cross platform tool that supports multiple regular expressions language, redet is an excellent candidate to consider. It does depend on having a tcl/tk runtime with the incr widgets library. The lowest friction path is to get ActiveTcl, but manually installing the incr Widgets (and incr tcl/tk) from the sourceforge download page is also pretty simple.
Not sure if this is exactly what you are looking for, but the most interesting regex tool I've found online in quite a long while is txt2re.com
There is RadSoftware's Regex Designer, it's very useful and is free but not open source ( no idea why).
The Regex Coach is excellent although it is not open source and Windows only, but there is an older version for Linux. Amusing note: it is written in Lisp. The above page also links to a Java version (written actually in Groovy, as it seems) which is open source.
JRegexpTester is written in Java, so portable.
REGex TESTER is yet another Web-based tester, it supports JavaScript RE syntax, of course, but also both PHP syntaxes (Perl style and Posix style).
Also Web based, Regular Expression Test Page to test Java syntax.
I have also a PCRE Workbench, written in Delphi, still for Windows...
According to a colleague of mine RegexBuddy works very well on different platforms using Wine: http://www.winehq.org/.
Unless you want to use only opensource software for philosophical reasons I would really recommend spending the $40 to buy a license--It will pay for itself in time saved.