Hello Poulomi,
If you know that you'll only ever have a single colon (:) character in the row, you can use that to split off the numeric part. Here's an example of how to do it with regular expressions:
s = '# Catchment area (km²): 122.300';
tok = regexp(s,'[^:]+:\s+([\S]+)','tokens');
str2double(tok{1}{1})
c = strsplit(s,':');
str2double(strtrim(c{2}))
-Cam