fix: support empty ref in source patterns

Allow patterns like 'repo@:path' where the ref between @ and : is empty.
This enables more lenient parsing where users can explicitly skip the ref
while still specifying a path.

Author: nadaverell

action.yml

@@ -74,13 +74,13 @@ runs:
           REPO="self"
           REF=""
           CONFIG_PATH="${BASH_REMATCH[1]}"
-        elif [[ "$SOURCE" =~ ^@([^:]+)(:.*)?$ ]]; then
-          # Just ref and optional path (e.g., "@main" or "@main:services/api" or "@main:")
+        elif [[ "$SOURCE" =~ ^@([^:]*)(:.*)?$ ]]; then
+          # Just ref and optional path (e.g., "@main" or "@main:services/api" or "@main:" or "@:path")
           REPO="self"
           REF="${BASH_REMATCH[1]}"
           CONFIG_PATH="${BASH_REMATCH[2]#:}"  # Remove : prefix
-        elif [[ "$SOURCE" =~ ^([^@:]+)(@[^:]+)?(:.*)?$ ]]; then
-          # Standard format with repo
+        elif [[ "$SOURCE" =~ ^([^@:]+)(@[^:]*)?(:.*)?$ ]]; then
+          # Standard format with repo (allows empty ref like "repo@:path")
           REPO="${BASH_REMATCH[1]}"
           REF="${BASH_REMATCH[2]#@}"   # Remove @ prefix
           CONFIG_PATH="${BASH_REMATCH[3]#:}"  # Remove : prefix

test-parsing.sh

@@ -19,11 +19,11 @@ test_pattern() {
         REPO="self"
         REF=""
         CONFIG_PATH="${BASH_REMATCH[1]}"
-    elif [[ "$SOURCE" =~ ^@([^:]+)(:.*)?$ ]]; then
+    elif [[ "$SOURCE" =~ ^@([^:]*)(:.*)?$ ]]; then
         REPO="self"
         REF="${BASH_REMATCH[1]}"
         CONFIG_PATH="${BASH_REMATCH[2]#:}"
-    elif [[ "$SOURCE" =~ ^([^@:]+)(@[^:]+)?(:.*)?$ ]]; then
+    elif [[ "$SOURCE" =~ ^([^@:]+)(@[^:]*)?(:.*)?$ ]]; then
         REPO="${BASH_REMATCH[1]}"
         REF="${BASH_REMATCH[2]#@}"
         CONFIG_PATH="${BASH_REMATCH[3]#:}"
@@ -67,6 +67,11 @@ test_pattern "self:" "SHOULD_PASS"
 test_pattern "Acme/configs:" "SHOULD_PASS"
 test_pattern "Acme/configs@main:" "SHOULD_PASS"
 
+# Empty ref patterns (missing ref value between @ and :)
+test_pattern "@:services/api" "SHOULD_PASS"
+test_pattern "acme-io/kubernetes@:platform-app" "SHOULD_PASS"
+test_pattern "Acme/configs@:" "SHOULD_PASS"
+
 # Other edge cases
 test_pattern ":" "SHOULD_PASS"
 test_pattern "@" "SHOULD_PASS"