grep with regex (-E) in Linux CLI - Step-by-Step Execution

Concept Flow - grep with regex (-E)

Start: Input file/text

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Run grep -E with regex

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grep reads each line

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Check if line matches regex

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Print line

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Next line

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End of file?

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Repeat

grep with -E reads each line of input and prints only those lines that match the extended regular expression.

Execution Sample

Linux CLI

grep -E 'cat|dog' animals.txt

Searches animals.txt and prints lines containing 'cat' or 'dog'.

Execution Table

Step	Line Content	Regex Match?	Action	Output
1	I have a cat	Yes	Print line	I have a cat
2	birds are flying	No	Skip line	I have a cat
3	My dog barks	Yes	Print line	I have a cat My dog barks
4	Fish swim	No	Skip line	I have a cat My dog barks
5	Cats and dogs	Yes	Print line	I have a cat My dog barks Cats and dogs
6	End of file	No	Stop	I have a cat My dog barks Cats and dogs

💡 Reached end of file, grep stops reading lines.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Current Line		I have a cat	birds are flying	My dog barks	Fish swim	Cats and dogs	End of file
Match Found		Yes	No	Yes	No	Yes	No
Output Lines		I have a cat	I have a cat	I have a cat My dog barks	I have a cat My dog barks	I have a cat My dog barks Cats and dogs	I have a cat My dog barks Cats and dogs

Key Moments - 3 Insights

Why does grep print some lines but not others?

grep prints only lines where the regex matches (see execution_table rows 1,3,5). Lines without a match are skipped (rows 2,4,6).

What does the -E option change in grep?

-E enables extended regex, allowing use of | for OR without escaping. This is why 'cat|dog' matches lines with either word.

Why is the output empty for some steps in the table?

When a line does not match, grep skips printing it, so output is empty for those steps (rows 2,4,6).

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 3?

A "I have a cat"

B "My dog barks"

C "I have a cat\nMy dog barks"

D "Cats and dogs"

💡 Hint

Check the Output column after step 3 in the execution_table.

At which step does grep first skip printing a line?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for the first 'No' in Regex Match? column and empty Output in execution_table.

If the regex was changed to 'cat|bird', which step's output would change?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Check which lines contain 'bird' and see their match status in execution_table.

Concept Snapshot

grep -E 'pattern' file
- Uses extended regex for matching
- Reads file line by line
- Prints lines matching regex
- Supports | for OR without escapes
- Stops at end of file

Full Transcript

This visual trace shows how grep with the -E option works. It reads each line of the input file and checks if the line matches the extended regular expression. If it matches, grep prints the line; if not, it skips it. The process repeats until the end of the file is reached. The example uses the regex 'cat|dog' to find lines containing either 'cat' or 'dog'. The execution table tracks each line, whether it matches, and the output produced. The variable tracker shows how the current line, match status, and output lines change step by step. Key moments clarify why some lines print and others don't, and how -E enables easier regex syntax. The quiz tests understanding of output at different steps and effects of changing the regex.