Is it possible to use regex pattern as a input parameter in a POST method?

it's possible. But not sure what your requirement is.

if you wanna input parameter with code in payload of POST, then you would use like below

// assume that a value is true or false according to your input
// assume 'a123' is your dynamical input below
// below value will be true
${=java.util.regex.Pattern.matches('(a|b)[0-9]+','a123')}

Hopefully it is an example for you to figure out what you required.

Thanks,

Aaron