regexp: fix slice bounds out of range panics

Regular expressions involving a (x){0} term are
simplified by removing this term from the
expression, just before the expression is compiled.

The number of subexpressions is evaluated before
the simplification. The number of capture instructions
in the compiled expressions is not necessarily in line
with the number of subexpressions.

When the ReplaceAll(String) methods are used, a number
of capture slots (nmatch) is evaluated as 2*(s+1)
(s being the number of subexpressions).

In some case, it can be higher than the number of capture
instructions evaluated at compile time, resulting in a
panic when the internal slices of regexp.machine
are resized to this value.

Fixed by capping the number of capture slots to the number
of capture instructions.

I must say I do not really see the benefits of setting
nmatch lower than re.prog.NumCap using this 2*(s+1) formula,
so perhaps this can be further simplified.

Fixes #11178
Fixes #11176

Change-Id: I21415e8ef2dd5f2721218e9a679f7f6bfb76ae9b
Reviewed-on: https://go-review.googlesource.com/14013Reviewed-by: Russ Cox <rsc@golang.org>

src/regexp/all_test.go

@@ -201,6 +201,12 @@ var replaceTests = []ReplaceTest{
 	// Substitution when subexpression isn't found
 	{"(x)?", "$1", "123", "123"},
 	{"abc", "$1", "123", "123"},
+
+	// Substitutions involving a (x){0}
+	{"(a)(b){0}(c)", ".$1|$3.", "xacxacx", "x.a|c.x.a|c.x"},
+	{"(a)(((b))){0}c", ".$1.", "xacxacx", "x.a.x.a.x"},
+	{"((a(b){0}){3}){5}(h)", "y caramb$2", "say aaaaaaaaaaaaaaaah", "say ay caramba"},
+	{"((a(b){0}){3}){5}h", "y caramb$2", "say aaaaaaaaaaaaaaaah", "say ay caramba"},
 }
 
 var replaceLiteralTests = []ReplaceTest{

src/regexp/regexp.go

@@ -482,6 +482,10 @@ func (re *Regexp) replaceAll(bsrc []byte, src string, nmatch int, repl func(dst
 	} else {
 		endPos = len(src)
 	}
+	if nmatch > re.prog.NumCap {
+		nmatch = re.prog.NumCap
+	}
+
 	for searchPos <= endPos {
 		a := re.doExecute(nil, bsrc, src, searchPos, nmatch)
 		if len(a) == 0 {