grep

Option	Function
-i	case-insensitve search (case-sensitive by default)
-v	exclude a certain pattern
-o	print only the matched pattern (line by default)
-q	quiet mode (no output, interested in the return status)
-E	extended regex (regex instead of basic patterns)

Multiple Patterns

grep "<pattern1>\|<pattern2>" <file>
# or
grep -e "<pattern1>" -e "<pattern2>" <file>

Substring

echo "<string>" | grep -q -E "<substring>"

PCRE

Extract whatever comes after Host: .

• o prints only the matched parts of the line and not the entire line.

• P activates Perl-compatible regular expressions (PCRE), in this case,\K:. This is a special feature in Perl-compatible regular expressions that resets the starting point of the match, that is, only the characters after \K will be included in the output.

grep -oP 'Host:\s*\K[^\s;]*' "${NMAP_FILE}"