Handle more escaped characters in regexps

spec/compiler/prepare_grammar/parse_regex_spec.cc

@@ -126,6 +126,16 @@ describe("parsing regex patterns", []() {
             })
         },
 
+        {
+            "escaped characters",
+            "\\t\\n\\r",
+            seq({
+                character({ '\t' }),
+                character({ '\n' }),
+                character({ '\r' }),
+            })
+        },
+
         {
             "plus repeats",
             "(ab)+(cd)+",

src/compiler/prepare_grammar/parse_regex.cc

@@ -174,6 +174,12 @@ class PatternParser {
         return CharacterSet({ { 'a', 'z' }, { 'A', 'Z' }, { '0', '9' } });
       case 'd':
         return CharacterSet({ { '0', '9' } });
+      case 't':
+        return CharacterSet({ '\t' });
+      case 'n':
+        return CharacterSet({ '\n' });
+      case 'r':
+        return CharacterSet({ '\r' });
       default:
         return CharacterSet({ value });
     }