Linux command: grep

How to use grep to match multiple strings in the same line?

grep 'string1|string2' filename

grep -E "string1|string2" filename

How can I grep for a string that begins with a dash/hyphen?

1. Quote AND/OR escape

Code:

 ls | grep "-a"

(that  is the escape character)

or

2. Use flag "--" to stop switch parsing

Code:

ls | grep -- -a