regexp/syntax: correctly print `^` BOL and `$` EOL

Fixes #12980.

Change-Id: I936db2f57f7c4dc80bb8ec32715c4c6b7bf0d708
Reviewed-on: https://go-review.googlesource.com/16112Reviewed-by: Russ Cox <rsc@golang.org>

src/regexp/syntax/regexp.go

@@ -166,9 +166,9 @@ func writeRegexp(b *bytes.Buffer, re *Regexp) {
 	case OpAnyChar:
 		b.WriteString(`(?s:.)`)
 	case OpBeginLine:
-		b.WriteRune('^')
+		b.WriteString(`(?m:^)`)
 	case OpEndLine:
-		b.WriteRune('$')
+		b.WriteString(`(?m:$)`)
 	case OpBeginText:
 		b.WriteString(`\A`)
 	case OpEndText:

src/regexp/syntax/simplify_test.go

@@ -19,8 +19,8 @@ var simplifyTests = []struct {
 	{`(ab)+`, `(ab)+`},
 	{`(ab)?`, `(ab)?`},
 	{`.`, `(?s:.)`},
-	{`^`, `^`},
-	{`$`, `$`},
+	{`^`, `(?m:^)`},
+	{`$`, `(?m:$)`},
 	{`[ac]`, `[ac]`},
 	{`[^ac]`, `[^ac]`},