[regex] fix uncaught exception when string is like "\\_"

[Zhenhang1213]

\_ is a valid identity escape in regular expressions, but previously libc++ incorrectly treated it as invalid. So I deleted this judgment
fixes #129062

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Austin (Zhenhang1213)

Changes

fixes #129062

---

Full diff: https://github.com/llvm/llvm-project/pull/129348.diff

1 Files Affected:

• (modified) libcxx/include/regex (+1-1)

diff --git a/libcxx/include/regex b/libcxx/include/regex
index 36ea55ce30921..6916f91628821 100644
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -3954,7 +3954,7 @@ _ForwardIterator basic_regex<_CharT, _Traits>::__parse_character_escape(
       ++__first;
       break;
     default:
-      if (*__first != '_' && !__traits_.isctype(*__first, ctype_base::alnum)) {
+      if (!__traits_.isctype(*__first, ctype_base::alnum)) {
         if (__str)
           *__str = *__first;
         else

[Zhenhang1213]

@frederick-vs-ja where could I find those tests？

[frederick-vs-ja]

@frederick-vs-ja where could I find those tests？

I think we can add a new case to libcxx/test/std/re/re.regex/re.regex.construct/ptr.pass.cpp.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[mordante]

Thanks for working on this. I've some comments.

[frederick-vs-ja]

Just a question: why didn't you rebase the branch onto the head of llvm:main while rebasing many times?

[Zhenhang1213]

Just a question: why didn't you rebase the branch onto the head of llvm:main while rebasing many times?

Right, I update it

[mordante]

Thanks, LGTM!

Do you need somebody to land this patch for you?