Regex: Make "match" and "sub" const member functions

Summary:
The Regex "match" and "sub" member functions were previously not "const"
because they wrote to the "error" member variable. This commit removes
those assignments, and instead assumes that the validity of the regex
is already known after the initial compilation of the regular
expression. As a result, these member functions were possible to make
"const". This makes it easier to do things like pre-compile Regexes
up-front, and makes "match" and "sub" thread-safe. The error status is
now returned as an optional output, which also makes the API of "match"
and "sub" more consistent with each other.

Also, some uses of Regex that could be refactored to be const were made const.

Patch by Nicolas Guillemot

Reviewers: jankratochvil, thopre

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67241

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372764 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Regex.cpp

@@ -52,14 +52,24 @@ Regex::~Regex() {
   }
 }
 
-bool Regex::isValid(std::string &Error) const {
-  if (!error)
-    return true;
+namespace {
 
+/// Utility to convert a regex error code into a human-readable string.
+void RegexErrorToString(int error, struct llvm_regex *preg,
+                        std::string &Error) {
   size_t len = llvm_regerror(error, preg, nullptr, 0);
 
   Error.resize(len - 1);
   llvm_regerror(error, preg, &Error[0], len);
+}
+
+} // namespace
+
+bool Regex::isValid(std::string &Error) const {
+  if (!error)
+    return true;
+
+  RegexErrorToString(error, preg, Error);
   return false;
 }
 
@@ -69,8 +79,14 @@ unsigned Regex::getNumMatches() const {
   return preg->re_nsub;
 }
 
-bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches){
-  if (error)
+bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches,
+                  std::string *Error) const {
+  // Reset error, if given.
+  if (Error && !Error->empty())
+    *Error = "";
+
+  // Check if the regex itself didn't successfully compile.
+  if (Error ? !isValid(*Error) : !isValid())
     return false;
 
   unsigned nmatch = Matches ? preg->re_nsub+1 : 0;
@@ -83,11 +99,13 @@ bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches){
 
   int rc = llvm_regexec(preg, String.data(), nmatch, pm.data(), REG_STARTEND);
 
+  // Failure to match is not an error, it's just a normal return value.
+  // Any other error code is considered abnormal, and is logged in the Error.
   if (rc == REG_NOMATCH)
     return false;
   if (rc != 0) {
-    // regexec can fail due to invalid pattern or running out of memory.
-    error = rc;
+    if (Error)
+      RegexErrorToString(error, preg, *Error);
     return false;
   }
 
@@ -112,14 +130,11 @@ bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches){
 }
 
 std::string Regex::sub(StringRef Repl, StringRef String,
-                       std::string *Error) {
+                       std::string *Error) const {
   SmallVector<StringRef, 8> Matches;
 
-  // Reset error, if given.
-  if (Error && !Error->empty()) *Error = "";
-
   // Return the input if there was no match.
-  if (!match(String, &Matches))
+  if (!match(String, &Matches, Error))
     return String;
 
   // Otherwise splice in the replacement string, starting with the prefix before