If you want to match an integer but only if it is followed by whitespace, do so directly:
-?[[:digit:]]+/[[:space:]]
That will fail if the integer is at the very end of the file without a newline, but text files are not supposed to end with anything other than a newline character. You can, however, do the following:
-?[[:digit:]]+/[[:space:]] { /* Handle an integer */ }
-?[[:digit:]]+/. { /* Handle the error */ }
-?[[:digit:]]+ { /* Handle an integer; this one must be at EOF */ }