You can use string and text functions in custom tables and fields. See Adding a New Custom Table and Defining and Editing Field Formula for further details. Below are explanations of available string and text functions.

ASCII(char)
Returns the Int ascii code of a given character.

CHAR(number)
Returns a character for a given ascii code.

CONCAT(string1,string2)
Concatenates string1 and string2.

INDEXOF(string1,string2,n)
Returns the index of the n-occurrence of string2 in string1.

INSERT(string1,string2,overrides,index)
Inserts string2 into string1 at a specified index, possibly overriding a specified number characters in string1.

INSERTSTR(string1,string2,index)
Inserts string2 into string1 at specified zero-based index.

ISNULLOREMPTY(string)
Returns true if the string is null or empty.

LEFT(string,length)
Returns a substring of a specified length, starting at the first character.

LENGTH(string)
Returns the length of a given string.

LOWER(string)
Returns a lowercase version of a given string.

LTRIM(string)
Removes whitespaces from the start of the string.

REMOTEAT(string,index)
Removes a character at a specified index from a given string.

REMOVERANGE(string,index,count)
Removes a specified number of characters from a given string starting at a specified index.

REPEAT(string,number)
Returns a concatenation of a string repeated a given number of time.

REPLACEALL(string,oldstring,newstring)
Replaces all occurrences of old string with new string.

RIGHT(string,length)
Returns a substring of a specified length, end with the last character.

RTRIM(string)
Removes whitespaces from the end of the string.

SOUNDEX(string)
Returns a four-character (SOUNDEX) code to evaluate the similarity of two objects.

STRBETWEEN(string, startstring, endstring)
Returns the substring contained between startstring and endstring.

STRPARTS(string,delimiter,n)
Returns the string delimited by a specified delimiter.  The search ends after the n-occurrence of the delimiter is found.

SUBSTRING(string,index,length)
Returns the substring of a specified length, starting at a specified index.

TOSTRING(object)
Converts a given value to a string.

TRIM(string)
Removes whitespaces from both ends of a string.

UPPER(string)
Returns an uppercase version of a given string.