there will be other ways, no doubt more elegant...

=--LEFT(A1,SUMPRODUCT(--(ISNUMBER(--MID(SUBSTITUTE(SUBSTITUTE(A1,".",0),",",0),ROW(INDIRECT("1:"&LEN(A1))),1)))))

where A1 holds string... note in the above I've had to make the assumption that "." & "," are valid numerical delimiters (eg 42.5 or 1,002 etc...)

If it's possible that you would have additional numbers beyond the value of interest (eg 42.5g 10a) you would need to adopt a different approach (where you look for the first text character)