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

"cc̈d̈d".replace(/[c̈d̈]/g, "X"); // result is "XXXXXX" and not expected "cXXd"

Compliant Solution

"cc̈d̈d".replace(/c̈|d̈/g, "X"); // result is "cXXd"