diff --git a/pygeoapi/util.py b/pygeoapi/util.py
index b0db50c..d7432a0 100644
--- a/pygeoapi/util.py
+++ b/pygeoapi/util.py
@@ -168,7 +168,7 @@ def yaml_load(fh: IO) -> dict:
     # # https://stackoverflow.com/a/55301129
 
     env_matcher = re.compile(
-        r'.*?\$\{(?P<varname>\w+)(:-(?P<default>[^}]+))?\}')
+        r'.*?\$\{(?P<varname>\w+)(:-(?P<default>[^}]*))?\}')
 
     def env_constructor(loader, node):
         result = ""
diff --git a/tests/pygeoapi-test-config-envvars.yml b/tests/pygeoapi-test-config-envvars.yml
index f84fc79..f8b7af2 100644
--- a/tests/pygeoapi-test-config-envvars.yml
+++ b/tests/pygeoapi-test-config-envvars.yml
@@ -31,7 +31,7 @@ server:
     bind:
         host: 0.0.0.0
         port: ${PYGEOAPI_PORT}
-    url: http://localhost:5000/
+    url: ${PYGEOAPI_URL:-http://localhost:5000/}
     mimetype: application/json; charset=UTF-8
     encoding: utf-8
     language: en-US
@@ -43,6 +43,8 @@ server:
     map:
         url: https://tile.openstreetmap.org/{z}/{x}/{y}.png
         attribution: '&copy; <a href="https://openstreetmap.org/copyright">OpenStreetMap contributors</a>'
+    api_rules:  # optional API design rules to which pygeoapi should adhere
+      url_prefix: ${PYGEOAPI_PREFIX:-}
 
 logging:
     level: DEBUG
diff --git a/tests/test_config.py b/tests/test_config.py
index 394a2bb..2b93a30 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -54,8 +54,19 @@ def test_config_envvars():
 
     assert isinstance(config, dict)
     assert config['server']['bind']['port'] == 5001
+    assert config['server']['url'] == 'http://localhost:5000/'
     assert config['metadata']['identification']['title'] == \
         'pygeoapi default instance my title'
+    assert config['server']['api_rules']['url_prefix'] == ''
+
+    os.environ['PYGEOAPI_URL'] = 'https://localhost:5000'
+    os.environ['PYGEOAPI_PREFIX'] = 'v1'
+
+    with open(get_test_file_path('pygeoapi-test-config-envvars.yml')) as fh:
+        config = yaml_load(fh)
+
+    assert config['server']['url'] == 'https://localhost:5000'
+    assert config['server']['api_rules']['url_prefix'] == 'v1'
 
     os.environ.pop('PYGEOAPI_PORT')