Based on these assumptions:
[li]/BOS exists in every string to be tested[/li][li]/BOS immediately follows the data you want returned[/li][li]The delimiter is ‘/’[/li][li]There is at least one delimiter before the data you’re looking for.[/li][li]There can be any number of delimiters in the string.[/li][li]The test string is in cell A1.[/li][li]The returned data is 99 characters or less.[/li][/ol]
Not pretty, but it will always return the string preceding ‘/BOS’
I suppose I could spoiler the explanation for those who want to figure it out for themselves, but you can just stop reading here if that’s the case. You know who you are.
Working from the inside out the way Excel will process it.
LEFT(A1,FIND("/BOS",A1)-1) strips off /BOS and everything after it.
SUBSTITUTE(…,"/",REPT(" ",99)) replaces all remaining instances of / with 99 spaces. This pads the data out to a minimum known length.
TRIM(RIGHT(…,99)) breaks off the rightmost 99 characters including the data you’re after and then removes whatever pad spaces that remained from the previous step.