GREP COMMAND BASIC REGEX EXAMPLES IN LINUX/UNIX

In this post, we will see how to use Basic regular expressions to increase the power of grep command.

Basic regular expressions:

^ -- Caret symbol, Match beginning of the line.

$ -- Match End of the line

* -- Match 0 or more occurrence of the previous character

. – Match Any single character

[] – Match Range of characters, just single occurrence.

            [a-z] –Match small letters

            [A-Z] –Match cap letters

            [0-9] – Match numerical.

[^] – Match Negate a sequence

 -- Match Escape character.

If we learn any new thing with the example, it will be there for a long time in our mind. Now we will see one example each regular expression what we have given above.

Example1: Find all the lines which start with “Mr”

grep ‘^Mr’ filename

Example2: Find all the lines which end with ‘sh’

grep ‘sh$’ filename

Example3: Display all the lines in a file expect empty lines.

grep –v ‘^$’ filename

Note:-v option is used to negate the search term, here ^$ indicates empty line, so our grep –v is filtering blank lines in the output.

Example4: Search for words which are bash, baash, bsh, baaash, baaaaash,

grep ‘ba*s’ filename

The above grep command will search for words which have a between b and s zero or more times.

Example5: Search for all words which start with b and h

grep ‘b.*h’ filename

Example6: Search for a word which is having three letters in it and starts with x and ends with m.

grep ‘x[a-z]m’ filename

This search will return all the three letter words which start with x and ends with m.

Example7: Search words do not contain ‘ac’ in a file.

grep ‘[^ac]’ filename

Example8: Search for a ‘[‘ in a file

Note: The “[“ is a special character, you cannot search with regular grep we have to use escape character () to negate it. So use ‘[‘ to search for [. This is applicable for all the special characters mentioned above.

grep ‘[’ filename

Please feel free to comment on this, if you have more thoughts. Stay tuned to our next grep post on how to use extended regular expressions.