[Day 4] 4. Grep (GNU Regular Expression Parser)

Definition: A collection/set/combination of characters used for searching or matching purposes.
Types of Characters:
1. Simple Characters: A-Z, a-z, 0-9
2. Special/Wildcard/Meta Characters:
```
.
```
  ,
```
*
```
  ,
```
+
```
  ,
```
?
```
  ,
```
()
```
  ,
```
[]
```
  ,
```
{}
```
  ,
```
\
```
  ,
```
|
```
  , etc.
  - Basic Wildcard: Matches a single character.
  - Extended Wildcard: Offers additional matching capabilities.
Examples of Regular Expressions:
- ```
a
```
- ```
a+b
```
- ```
a(123)c
```
- ```
ai{2}c
```

Syntax for Using
grep
:

Basic syntax:

grep "pattern" <filename>

grep [options] "pattern" <filename>

!4. Grep1.png

Common
grep
Options:

!4. Grep2.png

Examples of Using
grep
:

!4. Grep3.png

Escape Sequences:
- ```
grep "b*g" select.txt
```
  : Asterisk
```
*
```
  stands for 0 or more occurrences.
- ```
grep "b\*g" select.txt
```
  : Uses escape sequence to treat
```
*
```
  as a regular character.
- ```
grep "b[*]g" select.txt
```
  : Treats
```
*
```
  as a regular character.
- ```
grep -F "b*g" select.txt
```
  : Treats
```
*
```
  as a literal character due to
```
-F
```
  .
- ```
fgrep "b*g" select.txt
```
  : Similar function as above.

!4. Grep4.png

!4. Grep5.png

Exact Occurrences: Use
```
{}
```
for exact occurrences of a character.
Groups and Alternatives:
- ```
[]
```
  : Matches a single occurrence of any character within brackets.
- ```
()
```
  : Groups characters for combined matching.
- ```
|
```
  : Functions as an OR operator.

!4. Grep6.png !4. Grep7.png!4. Grep8.png