set someText to "abcdefgh0123456789"
set AppleScript's text item delimiters to {""} -- the default value
text items of someText --> {"a", "b", "c", "d", "e", "f", "g", "h", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"}
set AppleScript's text item delimiters to {"h0"}
text items of someText --> {"abcdefg", "123456789"}
set AppleScript's text item delimiters to {"pdq"}
text items of someText --> {"abcdefgh0123456789"}