The substring-before() function returns the part of a string that happens before the first occurrence of a specified substring.
string substring-before(string, substring)
Two strings (or, if the values passed in are not strings, their string-value). The first argument is the string to be searched and the second argument is the substring that marks the end of the result substring.
A substring of the first argument, up to, but not including, the first occurrence of the second argument substring. If the second argument does not occur, the function returns an empty string. If either argument is empty, the result is an empty string.
Defined: XPath, section 4.2.
