regexp: add HasMeta and regexp.Expr().

The former is a boolean function to test whether a string
contains a regular expression metacharacter; the second
returns the string used to compile the regexp.

R=gri, rsc
CC=golang-dev
https://golang.org/cl/3728041

src/pkg/regexp/all_test.go

@@ -269,6 +269,18 @@ func TestQuoteMeta(t *testing.T) {
 	}
 }
 
+func TestHasMeta(t *testing.T) {
+	for _, tc := range quoteMetaTests {
+		// HasMeta should be false if QuoteMeta returns the original string;
+		// true otherwise.
+		quoted := QuoteMeta(tc.pattern)
+		if HasMeta(tc.pattern) != (quoted != tc.pattern) {
+			t.Errorf("HasMeta(`%s`) = %t; want %t",
+				tc.pattern, HasMeta(tc.pattern), quoted != tc.pattern)
+		}
+	}
+}
+
 type numSubexpCase struct {
 	input    string
 	expected int

src/pkg/regexp/find_test.go

@@ -119,7 +119,11 @@ func build(n int, x ...int) [][]int {
 
 func TestFind(t *testing.T) {
 	for _, test := range findTests {
-		result := MustCompile(test.pat).Find([]byte(test.text))
+		re := MustCompile(test.pat)
+		if re.Expr() != test.pat {
+			t.Errorf("Expr() = `%s`; should be `%s`", re.Expr(), test.pat)
+		}
+		result := re.Find([]byte(test.text))
 		switch {
 		case len(test.matches) == 0 && len(result) == 0:
 			// ok

src/pkg/regexp/regexp.go

@@ -599,6 +599,11 @@ Loop:
 	re.prefix = string(b)
 }
 
+// Expr returns the source text used to compile the regular expression.
+func (re *Regexp) Expr() string {
+	return re.expr
+}
+
 // Compile parses a regular expression and returns, if successful, a Regexp
 // object that can be used to match against text.
 func Compile(str string) (regexp *Regexp, error os.Error) {
@@ -998,6 +1003,18 @@ func QuoteMeta(s string) string {
 	return string(b[0:j])
 }
 
+// HasMeta returns a boolean indicating whether the string contains
+// any regular expression metacharacters.
+func HasMeta(s string) bool {
+	// A byte loop is correct because all metacharacters are ASCII.
+	for i := 0; i < len(s); i++ {
+		if special(int(s[i])) {
+			return true
+		}
+	}
+	return false
+}
+
 // Find matches in slice b if b is non-nil, otherwise find matches in string s.
 func (re *Regexp) allMatches(s string, b []byte, n int, deliver func([]int)) {
 	var end int