Indeed++ Hence the "Hmmm?"

It can be dealt with:

for( qw[ ab abb abbb aabb aabbb aabbbb a_abb a_abbb abbaabbbaaabbbaxab
+b ] ) { 
    print "$_ contains '$1'" while  m[ 
        ( 
            (?<!a) (?{ local $a }) ( a (?{++$a}) )+ 
            (       ?{ local $b }) ( b (?{++$b}) )+ (?!b ) 
        )  
        (?(?{ $a+1 == $b }) (?=) | (?!) ) 
]gx };;

abb contains 'abb'
aabbb contains 'aabbb'
a_abb contains 'abb'
abbaabbbaaabbbaxabb contains 'abb'
abbaabbbaaabbbaxabb contains 'aabbb'
abbaabbbaaabbbaxabb contains 'abb'
[download]

But yours is a simpler approach, and probably quicker.

But the primary purpose of posting was to demonstrate the regex if...then...else construct--not to do someone else's systems testing for them.

Particularly as this is very unlikely to be the actual pattern they are trying to match