StringUtils: Add a utility for checking if a string is a valid regex

There's no function to check if a string is a valid regex, and doing it
via preg_match requires error suppression, often performed with @. This
isn't however a good practice, and suppressing errors via functions is
not a one line solution. Adding an utility function would probably be a
benefit for several use cases (try to grep '@preg_match' for a few of
them).

Change-Id: I257a096319f1ec13441e9f745dcd22545fdd5cc6

includes/libs/StringUtils.php

@@ -1,4 +1,7 @@
 <?php
+
+use Wikimedia\AtEase\AtEase;
+
 /**
  * Methods to play with strings.
  *
@@ -340,4 +343,21 @@ class StringUtils {
 			return new ArrayIterator( explode( $separator, $subject ) );
 		}
 	}
+
+	/**
+	 * Utility function to check if the given string is a valid regex. Avoids
+	 * manually calling suppressWarnings and restoreWarnings, and provides a
+	 * one-line solution without the need to use @.
+	 *
+	 * @since 1.34
+	 * @param string $string The string you want to check being a valid regex
+	 * @return bool
+	 */
+	public static function isValidRegex( $string ) {
+		AtEase::suppressWarnings();
+		// @phan-suppress-next-line PhanParamSuspiciousOrder False positive
+		$isValid = preg_match( $string, '' );
+		AtEase::restoreWarnings();
+		return $isValid !== false;
+	}
 }

tests/phpunit/includes/libs/StringUtilsTest.php

@@ -125,4 +125,28 @@ class StringUtilsTest extends PHPUnit\Framework\TestCase {
 			'noncharacters 2' => [ $PASS, "\xef\xbf\xbf" ],
 		];
 	}
+
+	/**
+	 * @param strin $input
+	 * @param bool $expected
+	 * @dataProvider provideRegexps
+	 * @covers StringUtils::isValidRegex
+	 */
+	public function testIsValidRegex( $input, $expected ) {
+		$this->assertSame( $expected, StringUtils::isValidRegex( $input ) );
+	}
+
+	/**
+	 * Data provider for testValidRegex
+	 */
+	public static function provideRegexps() {
+		return [
+			[ 'foo', false ],
+			[ '/foo/', true ],
+			[ '//', true ],
+			[ '/(foo/', false ],
+			[ '!(f[o]{2})!', true ],
+			[ '/foo\/', false ]
+		];
+	}
 }