egrep is a version of grep that supports extended regular expressions. egrep can be used to find all social security numbers in a file using a basic regex. All US social security numbers have the format: 123-45-6789. This can be broken into the regex containing a character class of three numeric digits, a dash, a character class of 2 numeric digits, a dash and finally a character class of four numeric digits.

egrep "[0-9]{3}[-][0-9]{2}[-][0-9]{4}" file

If the file contains social security numbers without dashes this regex will not match. To improve upon the regex you can use the ? operator that matches exactly 0 or 1 occurrence of the preceding character class.

egrep "[0-9]{3}[-]?[0-9]{2}[-]?[0-9]{4}" file

Placing the dash in its own character class is not required. I find that it makes the regex easier to read.

My next post will show you how to match a US telephone number using a slightly more complicated regular expression.