Grep
Basic
grep between two patterns
grep -oP '(?<=PATTERN1).*(?=PATTERN2)'
The regex pattern:  '(?<=PATTERN1).*(?=PATTERN2)'
Example:
To grep the content in between user:[ and ] rid: from users.txt file.
user:[Administrator] rid:[0x1f4]  
user:[Guest] rid:[0x1f5]          
user:[krbtgt] rid:[0x1f6]         
user:[audit2020] rid:[0x44f]      
cat users.txt | grep -oP '(?<=user:\[).*(?=\] rid:)'
grep between two patterns - include pattern
cat users.txt | grep -oP 'user:\[.*\] rid'
grep word of length
Return the words with length greater than 7 and less than 8
grep -o -w '\w\{7,8\}' user-list.txt