Fix a thinko in the parsing of substitutions in CommandObjectRegexCommand.

The old code incorrectly calculated the start position for the search
for the third (and subsequent) instance of a particular substitution
pattern (e.g. %1).

I also added a few test cases for this parsing covering this failure.

lldb/source/Commands/CommandObjectRegexCommand.cpp

@@ -43,7 +43,7 @@ bool CommandObjectRegexCommand::DoExecute(llvm::StringRef command,
                              percent_var, idx)) != std::string::npos;) {
             new_command.erase(percent_var_idx, percent_var_len);
             new_command.insert(percent_var_idx, match_str);
-            idx += percent_var_idx + match_str.size();
+            idx = percent_var_idx + match_str.size();
           }
         }
       }

lldb/test/API/commands/command/regex/TestRegexCommand.py

@@ -0,0 +1,31 @@
+"""
+This tests some simple examples of parsing regex commands
+"""
+
+import os
+import lldb
+import lldbsuite.test.lldbutil as lldbutil
+from lldbsuite.test.lldbtest import *
+
+
+class TestCommandRegexParsing(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def test_sample_rename_this(self):
+        """Try out some simple regex commands, make sure they parse correctly."""
+        self.runCmd("command regex one-substitution 's/(.+)/echo-cmd %1-first %1-second %1-third/'")
+        self.expect("one-substitution ASTRING",
+                    substrs = ["ASTRING-first", "ASTRING-second", "ASTRING-third"])
+
+        self.runCmd("command regex two-substitution 's/([^ ]+) ([^ ]+)/echo-cmd %1-first %2-second %1-third %2-fourth/'")
+        self.expect("two-substitution ASTRING BSTRING",
+                    substrs = ["ASTRING-first", "BSTRING-second", "ASTRING-third", "BSTRING-fourth"])
+        
+    def setUp(self):
+        # Call super's setUp().
+        TestBase.setUp(self)
+        self.runCmd("command script import " + os.path.join(self.getSourceDir(), "echo_command.py"))
+        self.runCmd("command script add echo-cmd -f echo_command.echo_command")

lldb/test/API/commands/command/regex/echo_command.py

@@ -0,0 +1,6 @@
+import lldb
+
+def echo_command(debugger, args, result, dict):
+    result.Print(args+'\n')
+    result.SetStatus(lldb.eReturnStatusSuccessFinishResult)
+    return True