Why can’t grep find negative numbers?

Suppose you're looking for instances of -42 in a file foo.txt. The command

 grep -42 foo.txt

won't work. Instead you'll get a warning message like the following.

 Usage: grep [OPTION]... PATTERN [FILE]... Try 'grep --help' for more information.

Putting single or double quotes around -42 won't help. The problem is that grep interprets 42 as a command line option, and doesn't have such an option. This is a problem if you're searching for negative numbers, or any pattern that beings with a dash, such as -able or --version.

The solution is to put -e in front of a regular expression containing a dash. That tells grep that the next token at the command line is a regular expression, not a command line option. So

 grep -e -42 foo.txt

will work.

You can also use -e several times to give grep several regular expressions to search for. For example,

 grep -e cat -e dog foo.txt

will search for "cat" and "dog."

See the previous post for another example of where grep doesn't seem to work. By default grep supports a restricted regular expression syntax and may need to be told to use "extended" regular expressions.