httpcaddyfile: Allow nameless regexp placeholder shorthand (#6113)

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

caddyconfig/httpcaddyfile/shorthands.go

@@ -33,7 +33,7 @@ func NewShorthandReplacer() ShorthandReplacer {
 		{regexp.MustCompile(`{path\.([\w-]*)}`), "{http.request.uri.path.$1}"},
 		{regexp.MustCompile(`{file\.([\w-]*)}`), "{http.request.uri.path.file.$1}"},
 		{regexp.MustCompile(`{query\.([\w-]*)}`), "{http.request.uri.query.$1}"},
-		{regexp.MustCompile(`{re\.([\w-]*)\.([\w-]*)}`), "{http.regexp.$1.$2}"},
+		{regexp.MustCompile(`{re\.([\w-\.]*)}`), "{http.regexp.$1}"},
 		{regexp.MustCompile(`{vars\.([\w-]*)}`), "{http.vars.$1}"},
 		{regexp.MustCompile(`{rp\.([\w-\.]*)}`), "{http.reverse_proxy.$1}"},
 		{regexp.MustCompile(`{err\.([\w-\.]*)}`), "{http.error.$1}"},

caddytest/integration/caddyfile_adapt/shorthand_parameterized_placeholders.caddyfiletest

@@ -1,5 +1,9 @@
 localhost:80
+
 respond * "{header.content-type} {labels.0} {query.p} {path.0} {re.name.0}"
+
+@match path_regexp ^/foo(.*)$
+respond @match "{re.1}"
 ----------
 {
 	"apps": {
@@ -22,6 +26,21 @@ respond * "{header.content-type} {labels.0} {query.p} {path.0} {re.name.0}"
 								{
 									"handler": "subroute",
 									"routes": [
+										{
+											"handle": [
+												{
+													"body": "{http.regexp.1}",
+													"handler": "static_response"
+												}
+											],
+											"match": [
+												{
+													"path_regexp": {
+														"pattern": "^/foo(.*)$"
+													}
+												}
+											]
+										},
 										{
 											"handle": [
 												{