regexp: fix prefix bug.
After a prefix match, the old code advanced the length of the
prefix. This is incorrect since the full match might begin
in the middle of the prefix. (Consider "aaaab+" matching
"aaaaaab").
Fixes #1373
R=rsc
CC=golang-dev
https://golang.org/cl/3795044
diff --git a/src/pkg/regexp/find_test.go b/src/pkg/regexp/find_test.go
index 34a7986..1690711 100644
--- a/src/pkg/regexp/find_test.go
+++ b/src/pkg/regexp/find_test.go
@@ -78,6 +78,7 @@
{`axxb$`, "axxcb", nil},
{`data`, "daXY data", build(1, 5, 9)},
{`da(.)a$`, "daXY data", build(1, 5, 9, 7, 8)},
+ {`zx+`, "zzx", build(1, 1, 3)},
// can backslash-escape any punctuation
{`\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\{\|\}\~`,
