Matching Parts of Strings with Regular Expressions in JavaScript

Question

Matching Parts of Strings with Regular Expressions in JavaScript

I've created an extensive regex pattern to identify URLs such as:

/^\/([^\/.?]+)(?:\/([^\/.?]+)(?:\/([^\/.?]+)(?:\.([^\/.?]+))?)?)?$/

It successfully matches:

/foo/bar/1.html

and returns ['foo', 'bar', '1', 'html']

As I work in Javascript, I am looking to extract the matched parts as the user types the URL (similar to a typeahead feature). For instance, if the user types:

/foo

The code should recognize that /foo has been matched, even though the entire regex pattern has not been satisfied. In Ruby, it's possible to retrieve an array containing only the partially matched elements like: ['foo', nil, nil, nil]. Is it also feasible, or straightforward, to achieve this in Javascript?

javascript regex

Answer 1

Answer №1

@minitech provided part of the solution: add a ? after each group in the regex to match even if they are missing. After that, check the groups in the regex result to determine which parts are matched and which are not.

For instance:

/^\/([^\/.?]+)?(?:\/([^\/.?]+)?(?:\/([^\/.?]+)?(?:\.([^\/.?]+))?)?)?$/.exec('/ab/c')

Would result in:

["/ab:c", "ab:c", "c", undefined, undefined]

If the fourth value is undefined, you can deduce which parts were entered or not.

Additionally, when working with complex regex patterns, it's easy to get confused about which group is which. To avoid this, I recommend using "named group" regular expressions. These are ordinary regex patterns that can be created using the XRegExp library (http://xregexp.com/), like this:

var result = XRegExp.exec('/ab/c', /^\/(?<fooPart>[^\/.?]+)?(?<barPart>?:\/([^\/.?]+)?(?:\/([^\/.?]+)?(?:\.([^\/.?]+))?)?)?$/
var fooPart = result.fooPart

This library also offers useful features like comments to help manage regular expressions effectively. It may be considered overkill if you only use one regex, but for extensive JS regexp tasks, I highly recommend this library.

Answer 2

@minitech provided part of the solution: add a ? after each group in the regex to match even if they are missing. After that, check the groups in the regex result to determine which parts are matched and which are not.

For instance:

/^\/([^\/.?]+)?(?:\/([^\/.?]+)?(?:\/([^\/.?]+)?(?:\.([^\/.?]+))?)?)?$/.exec('/ab/c')

Would result in:

["/ab:c", "ab:c", "c", undefined, undefined]

If the fourth value is undefined, you can deduce which parts were entered or not.

Additionally, when working with complex regex patterns, it's easy to get confused about which group is which. To avoid this, I recommend using "named group" regular expressions. These are ordinary regex patterns that can be created using the XRegExp library (http://xregexp.com/), like this:

var result = XRegExp.exec('/ab/c', /^\/(?<fooPart>[^\/.?]+)?(?<barPart>?:\/([^\/.?]+)?(?:\/([^\/.?]+)?(?:\.([^\/.?]+))?)?)?$/
var fooPart = result.fooPart

This library also offers useful features like comments to help manage regular expressions effectively. It may be considered overkill if you only use one regex, but for extensive JS regexp tasks, I highly recommend this library.

Matching Parts of Strings with Regular Expressions in JavaScript

Answer №1

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