A regex should never include a repetitive pattern whose body would match the empty string. This is usually a sign that a part of the regex is redundant or does not do what the author intended.

Noncompliant Code Example

"/(?:)*/"      // same as the empty regex, the '*' accomplishes nothing
"/(?:|x)*/"    // same as the empty regex, the alternative has no effect
"/(?:x|)*/"    // same as 'x*', the empty alternative has no effect
"/(?:x*|y*)*/" // same as 'x*', the first alternative would always match, y* is never tried
"/(?:x?)*/"    // same as 'x*'
"/(?:x?)+/"    // same as 'x*'

Compliant Solution

"/x*/"