To be a Limerick, a verse MUST have:
Lines one, two, and five MUST each have exactly three metric feet
Lines three and four MUST each have exactly two metric feet
The metric feet MUST be anapests ( da da DUM ) although the leading foot of each line may be an iamb ( da DUM) and the last foot of each line may have a trailing unaccented syllable ( da da DUM da). The classic Limerick is consistent in the use of iambs and trailing unaccented syllables, but this is not mandatory in recreational Limericks.
Lines one, two, and five MUST rhyme
Lines three and four MUST rhyme
A good Limerick will have a clever, unanticipated punch line as line five
A good Limerick will not be insipid or pointless
A good Limerick often has puns, word play, eccentric spelling, or some other witty feature
Any nonsense poem that lacks five lines, thirteen metric feet, or the aabba rhyme pattern is simply not a Limerick. It might be a sing-song or a la-de-da, but it’s not a Limerick.