IDS such as snort uses string match or PCRE as basis for the rules. A string rule will only match one pattern, a PCRE rule can match a list of patterns.
I wonder if hackers can use palindrome to defeat PCRE based rules. palindrome are symmetric patterns such as abccba uvwwvu
Since palindrome patterns can't be expressed by PCRE rules, so it's going to be hard to write a snort rule to detect it, right?