add isAffirmative() method

Author: Awilum

src/Strings.php

@@ -1708,6 +1708,16 @@ public function format(...$args): self
         return $this;
     }
 
+    /**
+     * Returns true if the string is affirmative, false otherwise.
+     *
+     * @return bool Returns TRUE on success or FALSE otherwise.
+     */
+    public function isAffirmative(): bool
+    {
+        return (bool) mb_ereg_match('^(?:1|t(?:rue)?|y(?:es)?|ok(?:ay)?)$', $this->string);
+    }
+
     /**
      * Returns true if the string is date and it is valid, false otherwise.
      *