#linux #cmd #grep #regex

grep_regex.md

Difference between grep with and without extended-regex

• by default grep interpret search pattern as basic regular expression i.e. the meta-characters ?, +, |, ( and ) are interpreted as literal, hence need to escape with backslash
• can pass in extended regular expresson option -E (or --extended-regex) which means it doesn't take the meta-characters literally hence do not need to escape with backslash

Anchoring

# a line begins with hello
grep '^hello' world.txt 

# a line ends with hello
grep 'hello$' world.txt

Single Character

# match anything begin with hello then 3 characters and ends with world
grep 'hello..world' sample.txt

Bracket Expression

# matches duck or dunk
grep 'du[cn]k' sample.txt

# do not include characters in bracket. 
# will match log lag but not leg
grep 'l[^e]g' sample.txt

# match a range of characters
# will match file-1..9
grep 'file-[1-9]' sample.txt

pre-defined characters

Quantifier	Character Classes
[:alnum:]	Alphanumeric characters.
[:alpha:]	Alphabetic characters.
[:blank:]	Space and tab.
[:digit:]	Digits.
[:lower:]	Lowercase letters.
[:upper:]	Uppercase letters.

Quantifiers

*	matches preceding 0 or more times
.*	any number of any characters
?	matches preceding 0 or none
+	matches preceding 1 or more times
{}	specify exact number with upper and lower bound

# match all integers that are between 3 to 9 digits
grep -E '[[:digit:]]{3,9} sample.txt

# basic regular expression
grep 'c\?at' sample.txt

# extended regular expression
grep -E 'c?at' sample.txt

Alternation (OR)

grep 'fatal\|error\|critical' /var/log/nginx/error.log 

grep -E 'fatal|error|critical' /var/log/nginx/error.log

Grouping patterns together

# matches careless and less
grep -E '(care)?less' sample.txt

Backslash Expression

Expression	Description
\b	Match a word boundary.
\<	Match an empty string at the beginning of a word.
\>	Match an empty string at the end of a word.
\w	Match a word.
\s	Match a space.

# matching heard or beard
grep '\b[hb]eard\b' sample.txt

References

https://linuxize.com/post/regular-expressions-in-grep/