How to Use Regex in Bash: Syntax, Examples, and Tips

In bash, you can use regex with the [[ string =~ regex ]] conditional expression to test if a string matches a pattern. The regex must be unquoted and follows extended regular expression syntax. Use this inside if statements or loops to automate pattern matching.

Syntax

The basic syntax to use regex in bash is:

• [[ string =~ regex ]]: Tests if string matches the regex.
• The regex is an extended regular expression and should not be quoted.
• If the match succeeds, the condition returns true (exit status 0).
• You can access matched groups using BASH_REMATCH array.

bash

[[ string =~ regex ]]

# Example:
if [[ "hello123" =~ [a-z]+[0-9]+ ]]; then
  echo "Match found"
fi

Example

This example checks if a variable contains a word followed by digits and prints the matched parts.

bash

#!/bin/bash
input="user42data"
if [[ $input =~ ([a-z]+)([0-9]+) ]]; then
  echo "Full match: ${BASH_REMATCH[0]}"
  echo "Letters: ${BASH_REMATCH[1]}"
  echo "Numbers: ${BASH_REMATCH[2]}"
else
  echo "No match"
fi

Output

Full match: user42
Letters: user
Numbers: 42

Common Pitfalls

Common mistakes when using regex in bash include:

• Quoting the regex pattern, which disables regex matching.
• Using single brackets [ ] instead of double brackets [[ ]] for regex tests.
• Expecting regex to behave like grep or sed without understanding bash's regex flavor.

Here is a wrong and right example:

bash

# Wrong: regex pattern quoted disables matching
input="abc123"
if [[ "$input" =~ "[a-z]+[0-9]+" ]]; then
  echo "Matched"
else
  echo "No match"
fi

# Right: regex unquoted
if [[ "$input" =~ [a-z]+[0-9]+ ]]; then
  echo "Matched"
else
  echo "No match"
fi

Output

No match
Matched

Quick Reference

Concept	Description	Example
Regex test	Check if string matches regex	[[ $str =~ regex ]]
Access groups	Get matched groups from regex	BASH_REMATCH[1], BASH_REMATCH[2]
No quotes	Do not quote regex pattern	[[ $str =~ [a-z]+ ]]
Double brackets	Use [[ ]] for regex, not [ ]	[[ $str =~ regex ]]
Exit status	0 if match, 1 if no match	if [[ $str =~ regex ]]; then ... fi

Key Takeaways

Use double brackets [[ ]] with =~ operator for regex matching in bash.

Do not quote the regex pattern; it must be unquoted for proper matching.

Access captured groups with the BASH_REMATCH array after a successful match.

Remember regex matching returns exit status 0 on success, 1 on failure.

Avoid using single brackets [ ] or quoting regex to prevent matching errors.