<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/68520>68520</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Is the behavior for llvm::Regex expected?
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          lcvon007
      </td>
    </tr>
</table>

<pre>
    ```
#include "llvm/Support/Regex.h"
#include "llvm/ADT/StringRef.h"
#include <string>
#include <iostream>
using namespace std;
int main() {
 string s("abs");
  llvm::StringRef reg(s);
  llvm::Regex r(reg);
 cout << (r.match("hal_abs") ? "match" : "not match" ) << endl;
 return 0;
}
```
It will output "match", and
```
#include <regex>
#include <string>
#include <iostream>
using namespace std;
int main() {
    std::string str="abs";
    std::regex r(str);
    cout << (regex_match("hal_abs", r) ? "matched" : "not match") << endl;
    return 0;
}
```
It will output not match.

Is it expected that the behavior is different between llvm and std? 
</pre>
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