egrep (Extended grep) in Unix

 You use egrep (Extended grep) in Unix when you need to search for patterns using extended regular expressions (ERE). It supports additional features like +, ?, |, and {} directly without escaping.

Example Use Cases:

1. Multiple Patterns:

   egrep 'apple|orange|banana' file.txt
   

   Matches any line containing "apple", "orange", or "banana".

2. Repetition:

   egrep 'a{2,}' file.txt
   

   Matches lines with "a" repeated 2 or more times.

3. Optional Characters:

   egrep 'colou?r' file.txt
   

   Matches "color" or "colour".

4. Find Lines with a Specific Range of Characters: egrep '[0-9]{3,5}' file.txt

          Matches lines containing a number with 3 to 5 digits (e.g., 123, 12345).

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Find Lines with Repeated Patterns:

egrep '(ab)+c' file.txt

Matches lines containing "ab" repeated one or more times, followed by "c" (e.g., "abc", "abababc").

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Match Words with Optional Characters:

egrep 'gr(e|a)y' file.txt

Matches "grey" or "gray".

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Search for Lines with a Specific Word or Another (Logical OR):

egrep 'cat|dog' file.txt
 or 
grep -E 'cat|dog' file.txt

Matches lines containing either "cat" or "dog".

Why Use egrep?

egrep is simply a shorthand for grep -E, so using it is more convenient for extended regular expressions. However, newer systems recommend grep -E because egrep is considered deprecated in some environments.

Use egrep when complex pattern matching is needed!