Fix missing flag from regex literal in corner case (#529)

In the specific corner case where a regex contains a flag but is not
followed by any symbols, the ast.RegExpLiteral.Literal value would drop
(part of) that flag.

This happens because the p.next() call during the parsing of the flag
wouldn't actually advance the p.chrOffset, and thus endOffset would be
erroneously adjusted.

The fix is just to use the current p.chrOffset and then call p.next()
afterwards.

parser/expression.go

@@ -136,8 +136,8 @@ func (p *parser) parseRegExpLiteral() *ast.RegExpLiteral {
 	flags := ""
 	if p.token == token.IDENTIFIER { // gim
 		flags = p.literal
+		endOffset = p.chrOffset
 		p.next()
-		endOffset = p.chrOffset - 1
 	}
 
 	var value string

parser/parser_test.go

@@ -975,6 +975,32 @@ func Test_parseNumberLiteral(t *testing.T) {
 	})
 }
 
+func Test_praseRegExpLiteral(t *testing.T) {
+	tt(t, func() {
+		test := func(input, literal, pattern, flags string) {
+			parser := newParser("", input, 1, nil)
+			program, err := parser.parse()
+			is(err, nil)
+
+			regex := program.Body[0].(*ast.ExpressionStatement).Expression.(*ast.RegExpLiteral)
+			is(regex.Literal, literal)
+			is(regex.Pattern, pattern)
+			is(regex.Flags, flags)
+		}
+
+		test("/abc/", "/abc/", "abc", "")
+		test("/abc/gim", "/abc/gim", "abc", "gim")
+		test("/abc/ ", "/abc/", "abc", "")
+		test("/abc/gim ", "/abc/gim", "abc", "gim")
+		test("/abc/;", "/abc/", "abc", "")
+		test("/abc/gim;", "/abc/gim", "abc", "gim")
+		test("/abc/\n", "/abc/", "abc", "")
+		test("/abc/gim\n", "/abc/gim", "abc", "gim")
+		test("/abc/;\n", "/abc/", "abc", "")
+		test("/abc/gim;\n", "/abc/gim", "abc", "gim")
+	})
+}
+
 func TestPosition(t *testing.T) {
 	tt(t, func() {
 		parser := newParser("", "// Lorem ipsum", 1, nil)