BASH - Files - Display every line in a file matching the regular expression

Display the lines matching the regular expression. (emulates grep).

awk '/regex_here/' filename

Display the lines that don't match the regular expression. (emulates grep -v).

awk '!/regex_here/' filename

Display the line before the regular expression match.

awk '/regex_here/{print x};{x=$0}' filename

Display the line after the regular expression match.

awk '/regex_here/{getline; print}' filename