add expectations for uppercase, lowercase, alpha and alphanumeric

tests/Features/Expect/toBeLowercase.php

@@ -0,0 +1,20 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect('lowercase')->toBeLowercase();
+    expect('UPPERCASE')->not->toBeLowercase();
+});
+
+test('failures', function () {
+    expect('UPPERCASE')->toBeLowercase();
+})->throws(ExpectationFailedException::class);
+
+test('failures with custom message', function () {
+    expect('UPPERCASE')->toBeLowercase('oh no!');
+})->throws(ExpectationFailedException::class, 'oh no!');
+
+test('not failures', function () {
+    expect('lowercase')->not->toBeLowercase();
+})->throws(ExpectationFailedException::class);