Finding problems with getting substring from a string.

Hi,
I am writing the substring function. But Some where I am going wrong.
I want a template which accepts the source string and key string as
parameters.
That template should select the string from the source which is befor the
key string.
eg.
Input paremeters
source string : OMLTree^123_UTI
Key String : ^

Output String : OMLTree

Can any one send the code for this template.

Thanks,
Laxmi Narayana