Linux Regular Expression Tutorial: Grep Regex Example

Types of Regular expressions

For ease of understanding let us learn the different types of Regex one by one.

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Basic Regular expressions

Some of the commonly used commands with Regular expressions are tr, sed, vi and grep. Listed below are some of the basic Regex.

Symbol	Descriptions
.	replaces any character
^	matches start of string
$	matches end of string
*	matches up zero or more times the preceding character
\	Represent special characters
()	Groups regular expressions
?	Matches up exactly one character

Let’s see an example.

Execute cat sample to see contents of an existing file

Search for content containing letter ‘a’.

‘^‘ matches the start of a string. Let’s search for content that STARTS with a

Only lines that start with character are filtered. Lines which do not contain the character ‘a’ at the start are ignored.

Let’s look into another example –

Select only those lines that end with t using $

Interval Regular expressions

These expressions tell us about the number of occurrences of a character in a string. They are

Expression	Description
{n}	Matches the preceding character appearing ‘n’ times exactly
{n,m}	Matches the preceding character appearing ‘n’ times but not more than m
{n, }	Matches the preceding character only when it appears ‘n’ times or more

Example:

Filter out all lines that contain character ‘p’

We want to check that the character ‘p’ appears exactly 2 times in a string one after the other. For this the syntax would be:

cat sample | grep -E p\{2}

Note: You need to add -E with these regular expressions.

Extended regular expressions

These regular expressions contain combinations of more than one expression. Some of them are:

Expression	Description
\+	Matches one or more occurrence of the previous character
\?	Matches zero or one occurrence of the previous character

Example:

Searching for all characters ‘t’

Suppose we want to filter out lines where character ‘a’ precedes character ‘t’

We can use command like

cat sample|grep "a\+t"

Brace expansion

The syntax for brace expansion is either a sequence or a comma separated list of items inside curly braces “{}”. The starting and ending items in a sequence are separated by two periods “..”.

Some examples:

In the above examples, the echo command creates strings using the brace expansion.

Summary:

• Regular expressions are a set of characters used to check patterns in strings
• They are also called ‘regexp’ and ‘regex’
• It is important to learn regular expressions for writing scripts
• Some basic regular expressions are:

Symbol	Descriptions
.	replaces any character
^	matches start of string
$	matches end of string

• Some extended regular expressions are:

Expression	Description
\+	Matches one or more occurrence of the previous character
\?	Matches zero or one occurrence of the previous character

• Some interval regular expressions are:

Expression	Description
{n}	Matches the preceding character appearing ‘n’ times exactly
{n,m}	Matches the preceding character appearing ‘n’ times but not more than m
{n, }	Matches the preceding character only when it appears ‘n’ times or more

• The brace expansion is used to generate strings. It helps in creating multiple strings out of one.