Anchor get_host_prefix regex to match Python behavior

Summary:
Python re.match() anchors at the start of the string, but Rust Regex::captures() finds a match anywhere. This caused get_host_prefix() to return "c" for IPv6-style hostnames like "8c50-153b-..." on Sandcastle TW hosts, while Python correctly returned empty string.

Add ^ anchor to the regex to match Python behavior.

Reviewed By: genevievehelsel

Differential Revision: D103090288

fbshipit-source-id: 9a34042a29c4ab7c5567bf0e24b678b9d5bc7967

Author: vilatto

eden/fs/cli_rs/edenfs-utils/src/hostname.rs

@@ -37,7 +37,7 @@ pub fn get_host_prefix(hostname: &str) -> String {
     //
     // Returns an empty string if no prefix is found.
 
-    let re = Regex::new(r"([a-zA-Z\-]+)\d+.*").unwrap();
+    let re = Regex::new(r"^([a-zA-Z\-]+)\d+.*").unwrap();
     re.captures(hostname).map_or_else(String::new, |captures| {
         captures.get(1).unwrap().as_str().to_string()
     })
@@ -85,5 +85,9 @@ mod tests {
 
         let mbp_prefix = get_host_prefix("helsel-mbp");
         assert_eq!(mbp_prefix, "");
+
+        let ipv6_style =
+            get_host_prefix("8c50-153b-0004-0000.twshared63901.04.rva3.tw.fbinfra.net");
+        assert_eq!(ipv6_style, "");
     }
 }