The Power of Regular Expression (Part 2 - grep/egrep)

Basic Usage 

egrep or grep -E

Run grep with extended regular expressions.

-i

Ignore case (ie uppercase, lowercase letters).

-v

Return all lines which don't match the pattern.

-w

Select only matches that form whole words.

-c

Print a count of matching lines.

Can be combined with the -v option to print a count of non matchine lines.

-l

Print the name of each file which contains a match.

Normally used when grep is invoked with wildcards for the file argument.

-n

Print the line number before each line that matches.

Regular Expression 

.

A single character

[abc]

Range. ie any one of these characters

[^abc]

Not range. A character that is not one of those enclosed.

(abc)

Group these characters and remember for later.

\n

Replace n with a number. Recall the charactes matched in that set of brackets.

May also be used to rename files or directories.

|

The logical 'or' operation.

\

In front of a character, removes it's special meaning.

Regular Expression Multiplier 

?

The preceding item is optional, it is matched zero or one times.

*

The preceding item will be matched zero or more times.

+

The preceding item will be matched one or more times.

{n}

The preceding item will be matched exactly n times.

{n,}

The preceding item will be matched n or more times.

{n,m}

The preceding item will be matched between n and m times.

Regular Expressions Anchors 

^

From the beginning of the line.

$

To the end of the line.

\<

At the beginning of a word.

\>

At the end of a word.

\b

Match either the beginning or end of a word.

Examples

egrep 'mellon' myfile.txt

Print every line in myfile.txt containing the string 'mellon'.

egrep -n 'mellon' myfile.txt

Same as above but print a line number before each line.

egrep '(.)bb\1' myfile.txt

Find every line with 2 b's and the same character both before and after those b's.

egrep -l '[0-9]{8,}' /files/projectx/*

Print each file in the directory projectx which contains a number of 8 digits or more.

egrep '\b[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,4}\b' myfile.txt

Print every line of myfiles.txt containing an email address.

Note: this is just a simple email matching pattern. There is a miniscule number of email addresses it will not match.