all: use bytes.Cut, strings.Cut

Many uses of Index/IndexByte/IndexRune/Split/SplitN
can be written more clearly using the new Cut functions.
Do that. Also rewrite to other functions if that's clearer.

For #46336.

Change-Id: I68d024716ace41a57a8bf74455c62279bde0f448
Reviewed-on: https://go-review.googlesource.com/c/go/+/351711
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

3 files changed, 16 insertions, 28 deletions

diff --git a/src/regexp/exec_test.go b/src/regexp/exec_test.go
index 1f9a7a96e0..5f8442668c 100644
--- a/src/regexp/exec_test.go
+++ b/src/regexp/exec_test.go
@@ -294,12 +294,9 @@ func parseResult(t *testing.T, file string, lineno int, res string) []int {
 				out[n] = -1
 				out[n+1] = -1
 			} else {
-				k := strings.Index(pair, "-")
-				if k < 0 {
-					t.Fatalf("%s:%d: invalid pair %s", file, lineno, pair)
-				}
-				lo, err1 := strconv.Atoi(pair[:k])
-				hi, err2 := strconv.Atoi(pair[k+1:])
+				loStr, hiStr, _ := strings.Cut(pair, "-")
+				lo, err1 := strconv.Atoi(loStr)
+				hi, err2 := strconv.Atoi(hiStr)
 				if err1 != nil || err2 != nil || lo > hi {
 					t.Fatalf("%s:%d: invalid pair %s", file, lineno, pair)
 				}
@@ -457,12 +454,11 @@ Reading:
 				continue Reading
 			}
 		case ':':
-			i := strings.Index(flag[1:], ":")
-			if i < 0 {
+			var ok bool
+			if _, flag, ok = strings.Cut(flag[1:], ":"); !ok {
 				t.Logf("skip: %s", line)
 				continue Reading
 			}
-			flag = flag[1+i+1:]
 		case 'C', 'N', 'T', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
 			t.Logf("skip: %s", line)
 			continue Reading
diff --git a/src/regexp/regexp.go b/src/regexp/regexp.go
index b547a2ab97..bfcf7910cf 100644
--- a/src/regexp/regexp.go
+++ b/src/regexp/regexp.go
@@ -922,23 +922,22 @@ func (re *Regexp) ExpandString(dst []byte, template string, src string, match []
 
 func (re *Regexp) expand(dst []byte, template string, bsrc []byte, src string, match []int) []byte {
 	for len(template) > 0 {
-		i := strings.Index(template, "$")
-		if i < 0 {
+		before, after, ok := strings.Cut(template, "$")
+		if !ok {
 			break
 		}
-		dst = append(dst, template[:i]...)
-		template = template[i:]
-		if len(template) > 1 && template[1] == '$' {
+		dst = append(dst, before...)
+		template = after
+		if template != "" && template[0] == '$' {
 			// Treat $$ as $.
 			dst = append(dst, '$')
-			template = template[2:]
+			template = template[1:]
 			continue
 		}
 		name, num, rest, ok := extract(template)
 		if !ok {
 			// Malformed; treat $ as raw text.
 			dst = append(dst, '$')
-			template = template[1:]
 			continue
 		}
 		template = rest
@@ -967,17 +966,16 @@ func (re *Regexp) expand(dst []byte, template string, bsrc []byte, src string, m
 	return dst
 }
 
-// extract returns the name from a leading "$name" or "${name}" in str.
+// extract returns the name from a leading "name" or "{name}" in str.
+// (The $ has already been removed by the caller.)
 // If it is a number, extract returns num set to that number; otherwise num = -1.
 func extract(str string) (name string, num int, rest string, ok bool) {
-	if len(str) < 2 || str[0] != '$' {
+	if str == "" {
 		return
 	}
 	brace := false
-	if str[1] == '{' {
+	if str[0] == '{' {
 		brace = true
-		str = str[2:]
-	} else {
 		str = str[1:]
 	}
 	i := 0
diff --git a/src/regexp/syntax/parse.go b/src/regexp/syntax/parse.go
index 7b4030935a..06a92fb3d7 100644
--- a/src/regexp/syntax/parse.go
+++ b/src/regexp/syntax/parse.go
@@ -824,13 +824,7 @@ func Parse(s string, flags Flags) (*Regexp, error) {
 				case 'Q':
 					// \Q ... \E: the ... is always literals
 					var lit string
-					if i := strings.Index(t, `\E`); i < 0 {
-						lit = t[2:]
-						t = ""
-					} else {
-						lit = t[2:i]
-						t = t[i+2:]
-					}
+					lit, t, _ = strings.Cut(t[2:], `\E`)
 					for lit != "" {
 						c, rest, err := nextRune(lit)
 						if err != nil {