Bash Script to Check if String is Alphabetic

Use [[ $string =~ ^[a-zA-Z]+$ ]] in Bash to check if a string is alphabetic; it returns true if the string contains only letters.

Examples

InputHelloWorld

OutputAlphabetic

InputHello123

OutputNot alphabetic

Input

OutputNot alphabetic

How to Think About It

To check if a string is alphabetic, think about verifying each character is a letter from A to Z or a to z. If all characters fit this rule and the string is not empty, then the string is alphabetic.

Algorithm

1

Get the input string.

2

Check if the string matches the pattern of only letters (a-z, A-Z).

3

If it matches, return that the string is alphabetic.

4

Otherwise, return that it is not alphabetic.

Code

bash

#!/bin/bash
read -p "Enter a string: " string
if [[ $string =~ ^[a-zA-Z]+$ ]]; then
  echo "Alphabetic"
else
  echo "Not alphabetic"
fi

Output

Enter a string: HelloWorld
Alphabetic

Dry Run

Let's trace the input 'Hello123' through the code

1

Input string

string = 'Hello123'

2

Pattern check

'Hello123' matches ^[a-zA-Z]+$ ? No

3

Output result

Print 'Not alphabetic'

Input String	Regex Match	Output
HelloWorld	Yes	Alphabetic
Hello123	No	Not alphabetic
	No	Not alphabetic

Why This Works

Step 1: Regex pattern

The pattern ^[a-zA-Z]+$ means the string must start and end with one or more letters only.

Step 2: Using [[ ]]

The Bash [[ ]] test allows regex matching with =~ operator.

Step 3: Conditional output

If the string matches, print 'Alphabetic'; otherwise, print 'Not alphabetic'.

Alternative Approaches

Using grep

bash

read -p "Enter a string: " string
echo "$string" | grep -qE '^[a-zA-Z]+$' && echo "Alphabetic" || echo "Not alphabetic"

Uses external grep command; slightly slower but works in older shells.

Using case statement

bash

read -p "Enter a string: " string
case "$string" in
  (*[!a-zA-Z]*) echo "Not alphabetic";;
  (*) echo "Alphabetic";;
esac

Uses shell pattern matching; simple and portable.

Complexity: O(n) time, O(1) space

Time Complexity

The regex check scans each character once, so time grows linearly with string length.

Space Complexity

No extra memory is needed besides the input string; the check is done in-place.

Which Approach is Fastest?

The built-in Bash regex [[ =~ ]] is fastest; grep calls an external process, and case uses shell pattern matching which is also efficient.

Approach	Time	Space	Best For
Bash regex [[ =~ ]]	O(n)	O(1)	Modern Bash scripts
grep command	O(n)	O(1)	Compatibility with older shells
case statement	O(n)	O(1)	Simple pattern matching without regex