substr string, pos, [n, replacement]
Extracts and returns a substring n characters long, starting at character position pos, from a given string. If pos is negative, the substring starts that far from the end of the string instead. If n is omitted, everything to the end of the string is returned. If n is negative, the length is calculated to leave that many characters off the end of the string.
You can use
substr() as an lvalue - replacing the delimited
substring with a new string - if string is given as an lvalue.
You can also specify a replacement string in the fourth parameter
to replace the substring. The original extracted substring is still returned.