caddy/caddytest/integration/caddyfile_adapt/replaceable_upstream.txt

*.sandbox.localhost {
	@sandboxPort {
		header_regexp first_label Host ^([0-9]{3})\.sandbox\.
	}
	handle @sandboxPort {
		reverse_proxy {re.first_label.1}
	}
	handle {
		redir {scheme}://application.localhost
	}
}

----------
{
	"apps": {
		"http": {
			"servers": {
				"srv0": {
					"listen": [
						":443"
					],
					"routes": [
						{
							"match": [
								{
									"host": [
										"*.sandbox.localhost"
									]
								}
							],
							"handle": [
								{
									"handler": "subroute",
									"routes": [
										{
											"group": "group2",
											"handle": [
												{
													"handler": "subroute",
													"routes": [
														{
															"handle": [
																{
																	"handler": "reverse_proxy",
																	"upstreams": [
																		{
																			"dial": "{http.regexp.first_label.1}"
																		}
																	]
																}
															]
														}
													]
												}
											],
											"match": [
												{
													"header_regexp": {
														"Host": {
															"name": "first_label",
															"pattern": "^([0-9]{3})\\.sandbox\\."
														}
													}
												}
											]
										},
										{
											"group": "group2",
											"handle": [
												{
													"handler": "subroute",
													"routes": [
														{
															"handle": [
																{
																	"handler": "static_response",
																	"headers": {
																		"Location": [
																			"{http.request.scheme}://application.localhost"
																		]
																	},
																	"status_code": 302
																}
															]
														}
													]
												}
											]
										}
									]
								}
							],
							"terminal": true
						}
					]
				}
			}
		}
	}
}