Preventing a capturing group from becoming "undefined" in JavaScript regex

Question

Preventing a capturing group from becoming "undefined" in JavaScript regex

Is there a way to capture text enclosed in either round brackets or square brackets using regular expressions more efficiently?

\[(.+)\]|\((.+)\)

Currently, when applying this regex to the examples "[test]" and "(test)", I get varying results such as "test" and "undefined". How can I modify the regex to only return "test" consistently?

(While this example regex demonstrates the issue, my actual regex is more complex yet faces the same dilemma.)

javascript regex capturing-group

Answer 1

Answer №1

If you employ a strategy of using look-ahead to match one option first, and then capturing it again in a second pass, you can successfully capture the desired text into a single capture group.

An alternative method involves utilizing additional captures:

(?=\[(.+?)\]|\((.+?)\))[(\[](\1\2)[)\]]

This technique works by initially matching either [...] or (...) with a look-ahead, and capturing the content between the delimiters into capture group 1 or 2. Then, it captures the same content again while disregarding the delimiter by referencing \1\2, leveraging back-referencing to achieve a non-participating match for an empty string. As a result, the identical string is captured into capture group 3, ensuring its participation.

This approach is believed to be reasonably efficient as the back-reference to the same position should yield quick results.

If this level of efficiency is insufficient and you require a RegExp with precisely one capture - representing the text enclosed within [..] or (..), consider utilizing look-behinds:

[(\[](.+?)(?:(?=\))(?<=\(\1)|(?=\])(?<=\[\1))

This pattern matches a [ or (, followed by a search for a capture positioned after it which is succeeded by either ) or ]. Subsequently, a reverse check confirms if the leading delimiter corresponds to the matching ( or [.

Although potentially less efficient, this method exclusively targets (...) and [...], capturing the contents between them within a singular capture group. In the event that the efficiency of the look-behind back-reference to the same position may vary, there's a possibility of it performing adequately. Conversely, if inefficiencies arise, it might entail extensive backward scanning (primarily during instances of sighting a prospective closing ) or ]).

This approach can also be transformed into a RegExp that isolates solely the desired text, presenting "capture zero" as the output (in conjunction with internal usage of capture 1), by aligning the initial [ or ( with a look-behind:

(?<=[(\[])(.+?)(?:(?=\))(?<=\(\1)|(?=\])(?<=\[\1))

(Both look-behinds and look-aheads continuously enrich the power of RegExps. They enable the repetition of matching the same sub-string multiple times through diverse RegExps, allowing subsequent expressions to reference captures from earlier matches.)

Answer 2

If you employ a strategy of using look-ahead to match one option first, and then capturing it again in a second pass, you can successfully capture the desired text into a single capture group.

An alternative method involves utilizing additional captures:

(?=\[(.+?)\]|\((.+?)\))[(\[](\1\2)[)\]]

This technique works by initially matching either [...] or (...) with a look-ahead, and capturing the content between the delimiters into capture group 1 or 2. Then, it captures the same content again while disregarding the delimiter by referencing \1\2, leveraging back-referencing to achieve a non-participating match for an empty string. As a result, the identical string is captured into capture group 3, ensuring its participation.

This approach is believed to be reasonably efficient as the back-reference to the same position should yield quick results.

If this level of efficiency is insufficient and you require a RegExp with precisely one capture - representing the text enclosed within [..] or (..), consider utilizing look-behinds:

[(\[](.+?)(?:(?=\))(?<=\(\1)|(?=\])(?<=\[\1))

This pattern matches a [ or (, followed by a search for a capture positioned after it which is succeeded by either ) or ]. Subsequently, a reverse check confirms if the leading delimiter corresponds to the matching ( or [.

Although potentially less efficient, this method exclusively targets (...) and [...], capturing the contents between them within a singular capture group. In the event that the efficiency of the look-behind back-reference to the same position may vary, there's a possibility of it performing adequately. Conversely, if inefficiencies arise, it might entail extensive backward scanning (primarily during instances of sighting a prospective closing ) or ]).

This approach can also be transformed into a RegExp that isolates solely the desired text, presenting "capture zero" as the output (in conjunction with internal usage of capture 1), by aligning the initial [ or ( with a look-behind:

(?<=[(\[])(.+?)(?:(?=\))(?<=\(\1)|(?=\])(?<=\[\1))

(Both look-behinds and look-aheads continuously enrich the power of RegExps. They enable the repetition of matching the same sub-string multiple times through diverse RegExps, allowing subsequent expressions to reference captures from earlier matches.)

Answer 3

Answer №2

Using Non-capturing Groups (?:) can help in excluding the captured characters within a regex expression. The code snippet below demonstrates how to extract text enclosed between () or [] without capturing it. (Note - This assumes that the desired text is always enclosed within [] or () and not part of a global search)

const regex = new RegExp(/\[(?:.+)\]|\((?:.+)\)/);
const string1 = 'foo[text1]bar';
const string2 = '(text2)foobar';
const extractedWord1 = regex.exec(string1)[0].slice(1,-1)
const extractedWord2 = regex.exec(string2)[0].slice(1,-1)

//extractedWord1: text1
//extractedWord2: text2

Answer 4

Using Non-capturing Groups (?:) can help in excluding the captured characters within a regex expression. The code snippet below demonstrates how to extract text enclosed between () or [] without capturing it. (Note - This assumes that the desired text is always enclosed within [] or () and not part of a global search)

const regex = new RegExp(/\[(?:.+)\]|\((?:.+)\)/);
const string1 = 'foo[text1]bar';
const string2 = '(text2)foobar';
const extractedWord1 = regex.exec(string1)[0].slice(1,-1)
const extractedWord2 = regex.exec(string2)[0].slice(1,-1)

//extractedWord1: text1
//extractedWord2: text2

Answer 5

Answer №3

If it doesn't matter which specific group numbers are captured, but only the text they contain, I believe the most straightforward approach is to filter the match afterward to eliminate any undefined groups:

for (const result of ' [foo] (bar) '.matchAll(/\[(.+)\]|\((.+)\)/g)) {
  const [, content] = result.filter(element => element !== undefined);
  console.log(content);
}

Answer 6

If it doesn't matter which specific group numbers are captured, but only the text they contain, I believe the most straightforward approach is to filter the match afterward to eliminate any undefined groups:

for (const result of ' [foo] (bar) '.matchAll(/\[(.+)\]|\((.+)\)/g)) {
  const [, content] = result.filter(element => element !== undefined);
  console.log(content);
}

Preventing a capturing group from becoming "undefined" in JavaScript regex

Answer №1

Answer №2

Answer №3

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