what is the actual regex pattern used in validation of Identifier (Entity ID) field in Basic SAML Configuration section in Microsoft azure ?

requirement : I have to write a code for validation of Entity ID through regex.
problem : Cant able to identify how the textbox validation is handled because the textbox excepts "a" and also it accepts "//hello/world?" but not this "hello/world?".

I may have to find all the possible inputs and test this textbox to find out the ideal regex
which is too much time consuming.
So, is it possible that i get the regex for it ?