[llvm] [Support] Add `\{<ref>}` backreferences in Regex::sub() (PR #67220)

[Igor Kudrin via llvm-commits]

https://github.com/igorkudrin updated https://github.com/llvm/llvm-project/pull/67220

>From 37de6364efecb6011d09122a789f2ed18c0f5f6b Mon Sep 17 00:00:00 2001
From: Igor Kudrin <ikudrin at accesssoftek.com>
Date: Fri, 22 Sep 2023 22:44:51 -0700
Subject: [PATCH] [Support] Add `\g<ref>` backreferences in Regex::sub()

The existing format of backreferences, `\<ref>`, does not allow digits
to be placed directly after the reference because they are included in
the reference number. The new format solves this problem by adding
explicit delimiters.
---
 llvm/include/llvm/Support/Regex.h    |  5 +++--
 llvm/lib/Support/Regex.cpp           | 19 +++++++++++++++++++
 llvm/unittests/Support/RegexTest.cpp | 16 ++++++++++++++++
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/llvm/include/llvm/Support/Regex.h b/llvm/include/llvm/Support/Regex.h
index ae4b9516f194e3a..bb7a8009b6bd0c3 100644
--- a/llvm/include/llvm/Support/Regex.h
+++ b/llvm/include/llvm/Support/Regex.h
@@ -85,8 +85,9 @@ namespace llvm {
                std::string *Error = nullptr) const;
 
     /// sub - Return the result of replacing the first match of the regex in
-    /// \p String with the \p Repl string. Backreferences like "\0" in the
-    /// replacement string are replaced with the appropriate match substring.
+    /// \p String with the \p Repl string. Backreferences like "\0" and "\g<1>"
+    /// in the replacement string are replaced with the appropriate match
+    /// substring.
     ///
     /// Note that the replacement string has backslash escaping performed on
     /// it. Invalid backreferences are ignored (replaced by empty strings).
diff --git a/llvm/lib/Support/Regex.cpp b/llvm/lib/Support/Regex.cpp
index dfbd373e4a98096..8fa71a749cc8e10 100644
--- a/llvm/lib/Support/Regex.cpp
+++ b/llvm/lib/Support/Regex.cpp
@@ -163,6 +163,25 @@ std::string Regex::sub(StringRef Repl, StringRef String,
 
     // FIXME: We should have a StringExtras function for mapping C99 escapes.
     switch (Repl[0]) {
+
+      // Backreference with the "\g<ref>" syntax
+    case 'g':
+      if (Repl.size() >= 4 && Repl[1] == '<') {
+        size_t End = Repl.find('>');
+        StringRef Ref = Repl.slice(2, End);
+        unsigned RefValue;
+        if (End != StringRef::npos && !Ref.getAsInteger(10, RefValue)) {
+          Repl = Repl.substr(End + 1);
+          if (RefValue < Matches.size())
+            Res += Matches[RefValue];
+          else if (Error && Error->empty())
+            *Error =
+                ("invalid backreference string 'g<" + Twine(Ref) + ">'").str();
+          break;
+        }
+      }
+      [[fallthrough]];
+
       // Treat all unrecognized characters as self-quoting.
     default:
       Res += Repl[0];
diff --git a/llvm/unittests/Support/RegexTest.cpp b/llvm/unittests/Support/RegexTest.cpp
index 78f37cdbd1ef89e..aecd2ff3133a3d7 100644
--- a/llvm/unittests/Support/RegexTest.cpp
+++ b/llvm/unittests/Support/RegexTest.cpp
@@ -127,6 +127,22 @@ TEST_F(RegexTest, Substitution) {
 
   EXPECT_EQ("aber", Regex("a[0-9]+b").sub("a\\100b", "a1234ber", &Error));
   EXPECT_EQ(Error, "invalid backreference string '100'");
+
+  EXPECT_EQ("012345", Regex("a([0-9]+).*").sub("0\\g<1>5", "a1234ber", &Error));
+  EXPECT_EQ("", Error);
+
+  EXPECT_EQ("0g<15", Regex("a([0-9]+).*").sub("0\\g<15", "a1234ber", &Error));
+  EXPECT_EQ("", Error);
+
+  EXPECT_EQ("0g<>15", Regex("a([0-9]+).*").sub("0\\g<>15", "a1234ber", &Error));
+  EXPECT_EQ("", Error);
+
+  EXPECT_EQ("0g<3e>1",
+            Regex("a([0-9]+).*").sub("0\\g<3e>1", "a1234ber", &Error));
+  EXPECT_EQ("", Error);
+
+  EXPECT_EQ("aber", Regex("a([0-9]+)b").sub("a\\g<100>b", "a1234ber", &Error));
+  EXPECT_EQ(Error, "invalid backreference string 'g<100>'");
 }
 
 TEST_F(RegexTest, IsLiteralERE) {