Linux Regular Expression Tutorial: Grep Regex Example
What are Linux Regular Expressions?
Linux Regular Expressions are special characters which help search data and matching complex patterns. Regular expressions are shortened as ‘regexp’ or ‘regex’. They are used in many Linux programs like grep, bash, rename, sed, etc.
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.
