add numeric string rule

Author: andersonjoseph

lib/rules/string.js

@@ -1,5 +1,7 @@
 "use strict";
 
+const NUMERIC_PATTERN = /^-?[0-9]\d*(\.\d+)?$/;
+
 module.exports = function checkString(value, schema) {
 	if (typeof value !== "string") {
 		return this.makeError("string");
@@ -9,7 +11,6 @@ module.exports = function checkString(value, schema) {
 	 	alpha: /^[a-zA-Z]+$/
 		alphaNum: /^[a-zA-Z0-9]+$/
 		alphaDash: /^[a-zA-Z0-9_\-]+$/
-
 	*/
 
 	const valueLength = value.length;
@@ -42,5 +43,9 @@ module.exports = function checkString(value, schema) {
 		return this.makeError("stringEnum", schema.enum);
 	}
 
+	if (schema.numeric === true && !RegExp(/^-?[1-9]\d*(\.\d+)?$/).test(value) ) {
+		return this.makeError("stringNumeric", 'A numeric string', value);
+	}
+
 	return true;
 };