Extended Regular Expressions Characters

 regular_express.txt

 

+ : Repeat the previous RE character of one or more.

Example : Search for "god" "good" "goood"... etc. strings. 

egrep -n 'go+d' regular_express.txt

 

 

? : Zero or one before the RE character.

Example: Search for the two strings "gd" and "god".

egrep -n 'go?d' --color=auto regular_express.txt 

| : Find out several strings by OR.

Example: Search for the two strings "gd" or "good".

egrep -n 'gd|good' --color=auto regular_express.txt

() :  Find group string.

Example: Search for the two strings "glad" or "good", because "g" and "d" are duplicates, so I can list "la" and "oo" in () and separate them with | 

egrep -n 'g(la|oo)d' --color=auto regular_express.txt

()+ :  Identification of multiple repeated groups.

Example: I'm looking for a string that starts with "A" and ends with "C", and there is more than one "xyz" string in the middle.

echo 'AxyzxyzxyzC' | egrep 'A(xyz)+C' --color=auto