[llvm] [FIX] Fix undefined-behaviour in regex engine. (PR #73071)

[via llvm-commits]

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@@ -146,7 +146,9 @@ matcher(struct re_guts *g, const char *string, size_t nmatch,
 	const char *stop;
 
 	/* simplify the situation where possible */
-	if (g->cflags&REG_NOSUB)
+        if (!string)
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tanmaysachan wrote:

I can move this to the function calling llvmregexec() within Regex.cpp ? Since the regex engine should be able to discard null strings as match-less for any regex.

https://github.com/llvm/llvm-project/pull/73071