// e.g. '[[:alnum:]][[:alnum:]][[:alnum:]]'. Please reference the laserfiche documentation for more information: https://www.laserfiche.com/support/webhelp/Laserfiche/10/en-US/administration/#../Subsystems/LFAdmin/Content/Restricting_Field_Data_to_a_Specific_Format.htm?Highlight=constraintformat
the corresponding regex
Given a string representing a laserfiche text constraint, returns the corresponding regex as string