Capture groups in Bash in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a capture group in Bash regular expressions?

A capture group is a part of a regular expression enclosed in parentheses. It saves the matched text so you can use it later in your script.

beginner

How do you access the text matched by a capture group in Bash?

You can access the matched text using the special array variable BASH_REMATCH. BASH_REMATCH[0] is the full match, BASH_REMATCH[1] is the first capture group, and so on.

beginner

Example: What does this Bash code print?

[[ "hello123" =~ ([a-z]+)([0-9]+) ]]
echo "${BASH_REMATCH[1]}"
echo "${BASH_REMATCH[2]}"

It prints: hello 123 The first capture group ([a-z]+) matches 'hello', the second ([0-9]+) matches '123'.

beginner

Why use capture groups in Bash scripts?

Capture groups let you extract parts of a string easily. For example, you can get a username from an email or a number from a filename without complicated commands.

beginner

What command structure is needed to use capture groups in Bash?

You use the double square brackets [[ ]] with the =~ operator to test a string against a regex with capture groups. Example:

[[ "$string" =~ regex ]]

In Bash, which variable holds the text matched by capture groups after a regex test?

A MATCH_GROUPS

B BASH_REMATCH

C REGEX_CAPTURE

D GROUP_MATCH

BASH_REMATCH is the special array variable that stores the full match and capture groups after using [[ string =~ regex ]].

What does BASH_REMATCH[0] contain after a regex match?

A The first capture group

B The regex pattern

C The entire matched string

D The last capture group

BASH_REMATCH[0] contains the entire string matched by the regex, not just a capture group.

Which operator is used inside [[ ]] to test a regex with capture groups in Bash?

A =~

B ==

C !=

D =>

The =~ operator is used to test if a string matches a regex pattern inside [[ ]].

How do you write a capture group in a Bash regex?

A [ ]

B { }

C < >

D ( )

Parentheses ( ) are used to create capture groups in regular expressions.

What will happen if you try to access BASH_REMATCH before a successful regex match?

A It will be empty or unset

B It will contain the previous match

C It will cause an error

D It will contain the regex pattern

BASH_REMATCH is only set after a successful regex match. Before that, it is empty or unset.

Explain how to use capture groups in Bash to extract parts of a string.

Think about how you test a string and then get parts of it.

Describe a simple real-life example where capture groups in Bash can help automate a task.

Imagine you want to get just a piece of text from a bigger string.