Answer 1

return /^\d{5,6}(-\d{4})?$/.test(str); ?

Answer 2

/(^\d{5}$)|(^\d{5}-\d{4}$)/

Answer 3

/^\d{6}(-\d{4})?$/

Simple replace 5 with 6 now it will allow 6 digit
ex :

231301-2123 //pass
618464 //Pass

if u want both
u can use 5 || 6 or 5,6

/^\d{5||6}(-\d{4})?$/
/^\d{5,6}(-\d{4})?$/

Answer 4

It's one of the concept taught in Sololearn itself. You can use {A,B} to specify the repeation range of a sub regex. I would like to memorize it as {N} is derived from the syntax.

https://www.sololearn.com/learn/9639/?ref=app

Answer 5

https://regexr.com

This might help u