List extensions in composer.json file.

“Note: It is important to list PHP extensions your project requires.”

See: https://getcomposer.org/doc/04-schema.md#package-links

Author: Maikuolan

composer.json

@@ -9,7 +9,8 @@
         "source": "https://github.com/phpMussel/SigTool"
     },
     "require": {
-        "php": ">=7.0.3"
+        "php": ">=7.0.3",
+        "ext-pcre": "*"
     },
     "suggest": {
         "php": ">=7.2.0 (this package performs significantly better with PHP >= 7.2.0).",