Regex in [[ ]] with =~ in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the operator =~ do inside [[ ]] in bash?

The =~ operator tests if the string on the left matches the regular expression on the right inside [[ ]]. It returns true if it matches, false otherwise.

beginner

How do you write a regex match to check if a variable contains only digits using [[ ]] and =~?

Use: [[ $var =~ ^[0-9]+$ ]] which means the variable contains one or more digits from start (^) to end ($).

intermediate

What happens if you put the regex in quotes when using =~ inside [[ ]]?

If the regex is quoted, it is treated as a fixed string, not a regex. This usually breaks the pattern matching. So, do NOT quote the regex.

intermediate

How can you access the part of the string matched by the regex after using =~?

Bash stores the matched groups in the array variable BASH_REMATCH. BASH_REMATCH[0] is the full match, BASH_REMATCH[1] the first group, and so on.

beginner

Why use [[ ]] with =~ instead of [ ] with grep or test?

[[ ]] with =~ is faster and simpler for regex matching inside bash scripts. It avoids spawning external commands like grep, making scripts more efficient.

In bash, what does [[ $var =~ ^abc ]] check?

A If $var equals 'abc'

B If $var starts with 'abc'

C If $var ends with 'abc'

D If $var contains 'abc' anywhere

The regex ^abc means the string starts with 'abc'.

What is stored in BASH_REMATCH[1] after a successful [[ $var =~ (pattern) ]] match?

A The variable name

B The full matched string

C The first captured group

D The regex pattern

BASH_REMATCH[1] holds the first captured group from the regex.

Which is correct to check if $var contains only letters using regex in [[ ]]?

A [[ $var =~ "^[a-zA-Z]+$" ]]

B [[ $var =~ '[a-zA-Z]+' ]]

C [[ $var == ^[a-zA-Z]+$ ]]

D [[ $var =~ ^[a-zA-Z]+$ ]]

Regex should NOT be quoted inside [[ ]] with =~ to work properly.

What does [[ $var =~ [0-9]{3} ]] return if $var='abc123def'?

A True, because '123' matches three digits

B False, because $var is not only digits

C Error, regex is invalid

D False, because regex must be quoted

The regex matches any three digits anywhere in the string.

Why avoid quoting the regex in [[ $var =~ regex ]]?

A Quoting disables regex matching and treats it as a string

B Quoting makes the script run faster

C Quoting is required for special characters

D Quoting changes the variable value

Quoting the regex disables pattern matching and treats it as a plain string.

Explain how to use regex matching inside [[ ]] with =~ in bash scripts.

Think about how bash tests patterns without external tools.

Describe common mistakes when using =~ with regex in bash and how to avoid them.

Focus on syntax and quoting rules.