You can use the following string functions in your fields' display text or in the values of your skip or validation logic. 



FunctionDescriptionExample
string(* arg)Converts another field type to a string (text).A number would need to be converted to string format in order to use it in a string function (such as to concatenate values).
concat(string_1, string_2, ...*)Joins the given strings together (without spaces).
=concat($firstname, " ", $lastname)
join(string separator, string_1, string_2, ...*)
Joins the values of a given set using the provided separator.
=join(" ", $firstname, $lastname)  or join(",", $select_field_1)
substr(string value, number start, number end?)Returns the portion of the string from the character at start index to end index - 1.=substr("2001-02-01", 0, 4) returns 2001
string-length(string arg)Returns the number of characters in the string.=string-length("12345") returns 5
contains(string haystack, string needle)Returns true if the first argument string contains the second argument string, and otherwise returns false.=contains("2018-09-01", "2018") returns true 
starts-with(string haystack, string needle)Returns true if the first argument string starts with the second argument string, and otherwise returns false.=starts-with("2018-09-01", "2018") returns true 
ends-with(string haystack, string needle)Returns true if the first argument string contains the second argument as a trailing substring.=ends-with("2018-09-01", "2018") returns false 
uuid()Returns a random 32 character identifier (RFC 4122 version 4 compliant).
Could return e.g. 081b9be5-fbb6-4807-9086-a26df958f861



Also see here for more functions and expressions you can use.