This weakness is not about regular expression complexity. Rather, it is about a regular expression that does not match all values that are intended. Consider the use of a regexp to identify acceptable values or to spot unwanted terms.

An overly restrictive regexp misses some potentially security-relevant values leading to either false positives *or* false negatives, depending on how the regexp is being used within the code. Consider the expression /[0-8]/ where the intention was /[0-9]/. This expression is not "complex" but the value "9" is not matched when maybe the programmer planned to check for it.