Remove Template Expression naming restrictions (#1139)

Fixes #1036

Relaxed the regular expression used to determine if an expression
is valid to support additional expression variable names.  We will
no longer restrict what an expression name can be.

core/src/main/java/feign/template/Expressions.java

@@ -38,7 +38,8 @@ public final class Expressions {
      *
      * see https://tools.ietf.org/html/rfc6570#section-2.3 for more information.
      */
-    expressions.put(Pattern.compile("(\\w[-\\w.\\[\\]]*[ ]*)(:(.+))?"), SimpleExpression.class);
+
+    expressions.put(Pattern.compile("^([+#./;?&]?)(.*)$"), SimpleExpression.class);
   }
 
   public static Expression create(final String value, final FragmentType type) {
@@ -68,10 +69,18 @@ public static Expression create(final String value, final FragmentType type) {
     Matcher matcher = expressionPattern.matcher(expression);
     if (matcher.matches()) {
       /* we have a valid variable expression, extract the name from the first group */
-      variableName = matcher.group(1).trim();
-      if (matcher.group(2) != null && matcher.group(3) != null) {
-        /* this variable contains an optional pattern */
-        variablePattern = matcher.group(3);
+      variableName = matcher.group(2).trim();
+      if (variableName.contains(":")) {
+        /* split on the colon */
+        String[] parts = variableName.split(":");
+        variableName = parts[0];
+        variablePattern = parts[1];
+      }
+
+      /* look for nested expressions */
+      if (variableName.contains("{")) {
+        /* nested, literal */
+        return null;
       }
     }
 

core/src/test/java/feign/template/QueryTemplateTest.java

@@ -174,4 +174,18 @@ public void expandCollectionValueWithBrackets() {
     /* brackets will be pct-encoded */
     assertThat(expanded).isEqualToIgnoringCase("collection%5B%5D=1,2");
   }
+
+  @Test
+  public void expandCollectionValueWithDollar() {
+    QueryTemplate template =
+        QueryTemplate.create(
+            "$collection",
+            Collections.singletonList("{$collection}"),
+            Util.UTF_8,
+            CollectionFormat.CSV);
+    String expanded =
+        template.expand(Collections.singletonMap("$collection", Arrays.asList("1", "2")));
+    /* brackets will be pct-encoded */
+    assertThat(expanded).isEqualToIgnoringCase("$collection=1,2");
+  }
 }