To be able to describe your problem more precisely and then to solve it you need studying the concept of regular expressions. There is something about this in the help-file (hope you installed it) and a lot of documents and tutorials in the web.
I now assume that you mean digits by “numbers”. If you are just interested in ONE (longest) or THE ONLY (longest) substring of your input that represents an INTEGER and there is no appearance of decimal or exponential parts. In this case a not too ambitious solution to your problem is possible by the use of regular expressions in SEARCH and with some auxiliary columns. A full parsing for numbers in different formats is much less easy. See books about formal languages and compilers for that!
A roughly designed example for the simple version you find in the attached file.