When placing Unicode Grapheme Clusters (characters which require to be encoded in
multiple Code Points) inside a character class of a regular expression, this will likely lead
to unintended behavior.
For instance, the grapheme cluster c̈ requires two code points: one for 'c', followed by one for the umlaut
modifier '\u{0308}'. If placed within a character class, such as [c̈], the regex will consider the character class being the
enumeration [c\u{0308}] instead. It will, therefore, match every 'c' and every umlaut that isn’t expressed as a
single codepoint, which is extremely unlikely to be the intended behavior.
This rule raises an issue every time Unicode Grapheme Clusters are used within a character class of a regular expression.
Noncompliant code example
preg_replace("/[c̈d̈]/", "X", "cc̈d̈d"); // Noncompliant, print "XXXXXX" instead of expected "cXXd".
Compliant solution
preg_replace("/c̈|d̈/", "X", "cc̈d̈d"); // print "cXXd"
