Where can I find unit tests for regular expressions in multiple languages?

Most of those languages are open source. Any decent project should have their test cases in said repo, otherwise I would be pretty concerned.

• Python's regex tests
• PHP's regex tests
• Perl's regex tests looks really extensive
• Open JDK's unit tests (an open source flavour of Java)
• Ruby's regex tests
• Mono's regex tests (open source version of .NET)
• .NET Core's regex tests
• RE2's tests (C++ regex engine developed at Google)
• C test suite (developed by AT&T Research)
• PCRE regex tests (Perl Compatible Regular Expressions C library)
• JavaScript regex tests (Ecma Technical Committee 39 compatability suite)

I also found an extensive chart on this page which might be of some help to you.

To have a complete list on one page, I've found the ones omitted from the accepted answer:

• Mono's regex tests (it's an open source version of .net)
• PHP's regex tests