From cc5410494e8f6d2af4b3b555420f452730899975 Mon Sep 17 00:00:00 2001
From: Solderpunk <solderpunk@sdf.org>
Date: Wed, 1 Jul 2020 11:13:38 +0200
Subject: [PATCH] Handle redirects using regular expressions, not just literal
 paths.

---
 README.md  | 16 ++++++++++------
 handler.go | 26 ++++++++++++++------------
 2 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/README.md b/README.md
index bb5222f..ecaccd6 100644
--- a/README.md
+++ b/README.md
@@ -206,12 +206,16 @@ directory listing:
 
 ### Redirects
 
-* `TempRedirects`: In this section of the config file, keys and values
-  are both path strings.  If the path component of a received request
-  matches one of the keys, Molly Brown will serve a redirect to the
-  corresponding value, using status code 30.  Note that currently
-  redirects cannot be specified using regular exressions, only literal
-  path strings.
+* `TempRedirects`: In this section of the config file, keys are
+  regular expressions which the server will attempt to match against
+  the path component if incoming request URLs.  If a match is found,
+  Molly Brown will serve a redirect to a new URL derived by replacing
+  the path component with the value corresponding to the matched key.
+  Within the replacement values, $1, $2, etc. will be replaced by the
+  first, second, etc. submatch in the regular expression.  Named
+  captures can also be used for more sophisticated redirect logic -
+  see the documentation for the Go standard library's `regexp` package
+  for full details.
 * `PermRedirects`: As per `TempRedirects` above, but Molly Brown will
   use the 31 status code instead of 30.
 
diff --git a/handler.go b/handler.go
index 61387d0..cf16c18 100644
--- a/handler.go
+++ b/handler.go
@@ -248,19 +248,21 @@ func parseMollyFiles(path string, config *Config, errorLogEntries chan string) {
 }
 
 func handleRedirects(URL *url.URL, config Config, conn net.Conn, log *LogEntry) {
-	for src, dst := range config.TempRedirects {
-		if URL.Path == src {
-			URL.Path = dst
-			conn.Write([]byte("30 " + URL.String() + "\r\n"))
-			log.Status = 30
-			return
+	handleRedirectsInner(URL, config.TempRedirects, 30, conn, log)
+	handleRedirectsInner(URL, config.PermRedirects, 31, conn, log)
+}
+
+func handleRedirectsInner(URL *url.URL, redirects map[string]string, status int, conn net.Conn, log *LogEntry) {
+	strStatus := strconv.Itoa(status)
+	for src, dst := range redirects {
+		compiled, err := regexp.Compile(src)
+		if err != nil {
+				continue
 		}
-	}
-	for src, dst := range config.PermRedirects {
-		if URL.Path == src {
-			URL.Path = dst
-			conn.Write([]byte("31 " + URL.String() + "\r\n"))
-			log.Status = 31
+		if compiled.MatchString(URL.Path) {
+			URL.Path = compiled.ReplaceAllString(URL.Path, dst)
+			conn.Write([]byte(strStatus + " " + URL.String() + "\r\n"))
+			log.Status = status
 			return
 		}
 	}