LibRegex: Ensure escaped code points are exactly 4 digits in length

Author: https://github.com/trflynn89
Commit: 2e4b6fd1ac
Pull-request: https://github.com/SerenityOS/serenity/pull/9367
Reviewed-by: https://github.com/linusg ✅

Tests/LibRegex/Regex.cpp

@@ -503,6 +503,8 @@ TEST_CASE(ECMA262_parse)
         { "(?", regex::Error::InvalidCaptureGroup },
         { "\\u1234", regex::Error::NoError, regex::ECMAScriptFlags::Unicode },
         { "[\\u1234]", regex::Error::NoError, regex::ECMAScriptFlags::Unicode },
+        { "\\u1", regex::Error::InvalidPattern, regex::ECMAScriptFlags::Unicode },
+        { "[\\u1]", regex::Error::InvalidPattern, regex::ECMAScriptFlags::Unicode },
         { ",(?", regex::Error::InvalidCaptureGroup }, // #4583
         { "{1}", regex::Error::InvalidPattern },
         { "{1,2}", regex::Error::InvalidPattern },