AppleScript: Index of substring in string

Question

I want to create a function that returns a substring of a specific string from the beginning of said string up to but not including the start of another specific string. Ideas?

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So something like:

substrUpTo(theStr, subStr)

so if I inputted substrUpTo("Today is my birthday", "my"), it would return a substring of the first argument up to but not including where the second argument begins. (i.e. it would return "Today is ")

Answer 1

set s to "Today is my birthday"
set AppleScript's text item delimiters to "my"
text item 1 of s
--> "Today is "

Answer 2

The built-in offset command should do it:

set s to "Today is my birthday"
log text 1 thru ((offset of "my" in s) - 1) of s
--> "Today is "