diff --git a/index.d.ts b/index.d.ts
index 1d5fd982c..991745726 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -43,13 +43,29 @@ export type Operator = ComparisonOperator | CombinationOperator | NegationOperat
  */
 export interface Filter extends Array<any> {}
 
+/**
+ * A FunctionFilter that expects a string (propertyName) as second arguement and
+ * a regular expression as third argument. An actual parser implementation has to
+ * return a value for this function expression.
+ */
+export interface StrMatchesFunctionFilter extends Filter {
+  0: 'FN_strMatches';
+  1: string;
+  2: RegExp;
+}
+
+/**
+ * A Filter that expresses a function.
+ */
+export type FunctionFilter = StrMatchesFunctionFilter;
+
 /**
  * A ComparisonFilter compares a value of an object (by key) with an expected
  * value.
  */
 export interface ComparisonFilter extends Filter {
   0: ComparisonOperator;
-  1: string;
+  1: string | FunctionFilter;
   2: string|number|boolean|null;
 }
 
diff --git a/sample.ts b/sample.ts
index b687d6636..4da6ea411 100644
--- a/sample.ts
+++ b/sample.ts
@@ -37,7 +37,7 @@ const sampleStyle: Style = {
     {
       name: 'Twelve year old Peter',
       filter: ['&&',
-        ['==', 'name', 'Peter'],
+        ['==', ['FN_strMatches', 'name', /Peter/], true],
         ['==', 'age', 12]
       ],
       scaleDenominator: {