@anhnha You've got three nested lists

@anhnha consider that {a} looks like a vector (albeit with a single element), so has dimension 1. Then {{a}} looks like a matrix (albeit with a single element) so has dimensions 2. Then {{{a}}} has 3 dimensions, {{{{a}}}} has 4, etc.

Essentially I want a function where the user input is a pattern, and the function needs to check whether this pattern itself follows a specific paradigm

@AntiEarth I mean you can program this up

Pattern matching on patterns should definitely be possible, they're just expressions like everything else, after all.

I naturally think it's possible in principle, but that doesn't lend much help for understanding how to do it. E.g. how to delimit what, in a pattern-of-patterns, is a fixture (of the attemptedly matched pattern) and what is a "top-level" pattern

I take it there are no functions dedicated to making this distinction clearer/easier than (which was my question)

but my intent is actually something slightly harder

which is; I want to, given a pattern as user input, generate all expressions that are instances of that pattern (under some additional constraints). The additional constraints are patterns themselves. So sort of I want to generate every expression that satisfies two patterns

(clearly the constraints will have to restrict the space to finite matches)

@AntiEarth Concerning patterns, are you aware of Verbatim ?

f[Verbatim[_Integer]] := ok1;
f[_Integer]

--> ok1

f[6]

--> f[6]

(I have Discovered Verbatim in Roman Maeder's book "the mathematica programmer II", in the chapter concerning a Prolog Interpreter implemented with Mathematica.)

@andre314 @AntiEarth you'll be doing a lot of Verbatim. Also look up Pattern and HoldPattern so you understand its many, many variants