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

[via llvm-commits]

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@@ -82,8 +82,8 @@ unsigned Regex::getNumMatches() const {
 
 bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches,
                   std::string *Error) const {
-  // Exit without match if string is empty
-  if (String.empty())
+  // No match for null string data
+  if (String.data() == nullptr)
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tanmaysachan wrote:

I believe the empty() checks were failing because a size 0 string and null string both were returning true for it. And an empty regex can match with an empty string?

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