Returns the substring of the first argument string that precedes the first occurrence of the second argument string in the first argument string, or the empty string if the first argument string does not contain the second argument string.
string substring-before(string, string)
If an argument is not of type string, it is first converted to a string and then evaluated.
The following function call returns "1999":
substring-before("1999/04/01","/")
The following function call returns an empty string "":
substring-before("1999/04/01","2")
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