Anchors (^, $) in Bash Scripting - Step-by-Step Execution

Concept Flow - Anchors (^, $)

Start

↓

Input String

↓

Apply ^ anchor

↓

Match start of string?

No→No match

Yes↓

Apply $ anchor

↓

Match end of string?

No→No match

Yes↓

Full match found

↓

End

Anchors ^ and $ check if a pattern matches the start or end of a string in bash regex matching.

Execution Sample

Bash Scripting

echo "hello" | grep -E '^he'
echo "hello" | grep -E 'lo$'

Check if 'hello' starts with 'he' and ends with 'lo' using anchors.

Execution Table

Step	Input String	Pattern	Anchor Used	Match Result	Output
1	hello	^he	^	Matches start	hello
2	hello	lo$	$	Matches end	hello
3	hello	^lo	^	No match
4	hello	he$	$	No match

💡 No more patterns to test; matching ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
Input String	hello	hello	hello	hello	hello
Pattern		^he	lo$	^lo	he$
Match Result		Matches start	Matches end	No match	No match
Output		hello	hello

Key Moments - 2 Insights

Why does '^he' match 'hello' but '^lo' does not?

Because '^' means match only at the start of the string. 'hello' starts with 'he' but not with 'lo' (see execution_table rows 1 and 3).

Why does 'lo$' match 'hello' but 'he$' does not?

'$' means match only at the end of the string. 'hello' ends with 'lo' but not with 'he' (see execution_table rows 2 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the output when pattern '^he' is used on 'hello'?

A "hello"

B "" (empty)

C "he"

D "lo"

💡 Hint

Check row 1 in execution_table under Output column.

At which step does the pattern fail to match the input string?

A Step 1

B Step 2

C Step 3

D No failure

💡 Hint

Look for 'No match' in Match Result column in execution_table.

If we change the pattern from '^he' to '^h', what would happen at Step 1?

A No match

B Matches start

C Matches end

D Matches whole string

💡 Hint

'^h' still matches start of 'hello' as 'hello' starts with 'h' (see concept of ^ anchor).

Concept Snapshot

Anchors ^ and $ in bash regex check string positions.
^ matches start of string.
$ matches end of string.
Use with grep -E for pattern matching.
Example: '^he' matches strings starting with 'he'.
Example: 'lo$' matches strings ending with 'lo'.

Full Transcript

Anchors ^ and $ are special symbols in bash regex that let you check if a pattern is at the start or end of a string. The ^ anchor means the pattern must be at the very beginning. The $ anchor means the pattern must be at the very end. For example, using grep -E '^he' on the string 'hello' matches because 'hello' starts with 'he'. Using 'lo$' matches because 'hello' ends with 'lo'. If the pattern does not match at the required position, no output is returned. This helps you find exact matches at string edges.