Capturing groups
A part of a pattern can be enclosed in parentheses (. ) . This is called a “capturing group”.
- It allows to get a part of the match as a separate item in the result array.
- If we put a quantifier after the parentheses, it applies to the parentheses as a whole.
Examples
Let’s see how parentheses work in examples.
Example: gogogo
Without parentheses, the pattern go+ means g character, followed by o repeated one or more times. For instance, goooo or gooooooooo .
Parentheses group characters together, so (go)+ means go , gogo , gogogo and so on.
alert( 'Gogogo now!'.match(/(go)+/ig) ); // "Gogogo"Example: domain
Let’s make something more complex – a regular expression to search for a website domain.
mail.com users.mail.com smith.users.mail.comAs we can see, a domain consists of repeated words, a dot after each one except the last one.
In regular expressions that’s (\w+\.)+\w+ :
let regexp = /(\w+\.)+\w+/g; alert( "site.com my.site.com".match(regexp) ); // site.com,my.site.comThe search works, but the pattern can’t match a domain with a hyphen, e.g. my-site.com , because the hyphen does not belong to class \w .
We can fix it by replacing \w with [\w-] in every word except the last one: ([\w-]+\.)+\w+ .
Example: email
The previous example can be extended. We can create a regular expression for emails based on it.
The email format is: name@domain . Any word can be the name, hyphens and dots are allowed. In regular expressions that’s [-.\w]+ .
let regexp = /[-.\w]+@([\w-]+\.)+[\w-]+/g; alert("my@mail.com @ his@site.com.uk".match(regexp)); // my@mail.com, his@site.com.ukThat regexp is not perfect, but mostly works and helps to fix accidental mistypes. The only truly reliable check for an email can only be done by sending a letter.
Parentheses contents in the match
Parentheses are numbered from left to right. The search engine memorizes the content matched by each of them and allows to get it in the result.
The method str.match(regexp) , if regexp has no flag g , looks for the first match and returns it as an array:
- At index 0 : the full match.
- At index 1 : the contents of the first parentheses.
- At index 2 : the contents of the second parentheses.
- …and so on…
For instance, we’d like to find HTML tags <.*?>, and process them. It would be convenient to have tag content (what’s inside the angles), in a separate variable.
Let’s wrap the inner content into parentheses, like this: <(.*?)>.
Now we’ll get both the tag as a whole and its contents h1 in the resulting array:
let str = 'Hello, world!
'; let tag = str.match(/<(.*?)>/); alert( tag[0] ); // alert( tag[1] ); // h1
Nested groups
Parentheses can be nested. In this case the numbering also goes from left to right.
For instance, when searching a tag in we may be interested in:
- The tag content as a whole: span >.
- The tag name: span .
- The tag attributes: class=»my» .
Let’s add parentheses for them: <(([a-z]+)\s*([^>]*))> .
Here’s how they are numbered (left to right, by the opening paren):
let str = ']*))>/; let result = str.match(regexp); alert(result[0]); //The zero index of result always holds the full match.
Then groups, numbered from left to right by an opening paren. The first group is returned as result[1] . Here it encloses the whole tag content.
Then in result[2] goes the group from the second opening paren ([a-z]+) – tag name, then in result[3] the tag: ([^>]*) .
The contents of every group in the string:
Optional groups
Even if a group is optional and doesn’t exist in the match (e.g. has the quantifier (. )? ), the corresponding result array item is present and equals undefined .
For instance, let’s consider the regexp a(z)?(c)? . It looks for "a" optionally followed by "z" optionally followed by "c" .
If we run it on the string with a single letter a , then the result is:
let match = 'a'.match(/a(z)?(c)?/); alert( match.length ); // 3 alert( match[0] ); // a (whole match) alert( match[1] ); // undefined alert( match[2] ); // undefinedThe array has the length of 3 , but all groups are empty.
And here’s a more complex match for the string ac :
let match = 'ac'.match(/a(z)?(c)?/) alert( match.length ); // 3 alert( match[0] ); // ac (whole match) alert( match[1] ); // undefined, because there's nothing for (z)? alert( match[2] ); // cThe array length is permanent: 3 . But there’s nothing for the group (z)? , so the result is ["ac", undefined, "c"] .
Searching for all matches with groups: matchAll
The method matchAll is not supported in old browsers.
When we search for all matches (flag g ), the match method does not return contents for groups.
For example, let’s find all tags in a string:
let str = ' '; let tags = str.match(/<(.*?)>/g); alert( tags ); // ,The result is an array of matches, but without details about each of them. But in practice we usually need contents of capturing groups in the result.
To get them, we should search using the method str.matchAll(regexp) .
It was added to JavaScript language long after match , as its “new and improved version”.
Just like match , it looks for matches, but there are 3 differences:
- It returns not an array, but an iterable object.
- When the flag g is present, it returns every match as an array with groups.
- If there are no matches, it returns not null , but an empty iterable object.
let results = ' '.matchAll(/<(.*?)>/gi); // results - is not an array, but an iterable object alert(results); // [object RegExp String Iterator] alert(results[0]); // undefined (*) results = Array.from(results); // let's turn it into array alert(results[0]); // ,h1 (1st tag) alert(results[1]); // ,h2 (2nd tag)As we can see, the first difference is very important, as demonstrated in the line (*) . We can’t get the match as results[0] , because that object isn’t pseudoarray. We can turn it into a real Array using Array.from . There are more details about pseudoarrays and iterables in the article Iterables.
There’s no need in Array.from if we’re looping over results:
let results = ' '.matchAll(/<(.*?)>/gi); for(let result of results) < alert(result); // first alert: ,h1 // second: ,h2 >let [tag1, tag2] = ' '.matchAll(/<(.*?)>/gi);Every match, returned by matchAll , has the same format as returned by match without flag g : it’s an array with additional properties index (match index in the string) and input (source string):
let results = ' '.matchAll(//gi); let [tag1, tag2] = results; alert( tag1[0] ); // alert( tag1[1] ); // h1 alert( tag1.index ); // 0 alert( tag1.input ); //
Why is the method designed like that? The reason is simple – for the optimization.
The call to matchAll does not perform the search. Instead, it returns an iterable object, without the results initially. The search is performed each time we iterate over it, e.g. in the loop.
So, there will be found as many results as needed, not more.
E.g. there are potentially 100 matches in the text, but in a for..of loop we found 5 of them, then decided it’s enough and made a break . Then the engine won’t spend time finding other 95 matches.
Named groups
Remembering groups by their numbers is hard. For simple patterns it’s doable, but for more complex ones counting parentheses is inconvenient. We have a much better option: give names to parentheses.
That’s done by putting ? immediately after the opening paren.
For example, let’s look for a date in the format “year-month-day”:
let dateRegexp = /(?5)-(?8)-(?7)/; let str = "2019-04-30"; let groups = str.match(dateRegexp).groups; alert(groups.year); // 2019 alert(groups.month); // 04 alert(groups.day); // 30As you can see, the groups reside in the .groups property of the match.
To look for all dates, we can add flag g .
We’ll also need matchAll to obtain full matches, together with groups:
let dateRegexp = /(?6)-(?9)-(?8)/g; let str = "2019-10-30 2020-01-01"; let results = str.matchAll(dateRegexp); for(let result of results) < let = result.groups; alert(`$.$.$`); // first alert: 30.10.2019 // second: 01.01.2020 >Capturing groups in replacement
Method str.replace(regexp, replacement) that replaces all matches with regexp in str allows to use parentheses contents in the replacement string. That’s done using $n , where n is the group number.
let str = "John Bull"; let regexp = /(\w+) (\w+)/; alert( str.replace(regexp, '$2, $1') ); // Bull, JohnFor named parentheses the reference will be $ .
For example, let’s reformat dates from “year-month-day” to “day.month.year”:
let regexp = /(?6)-(?4)-(?5)/g; let str = "2019-10-30, 2020-01-01"; alert( str.replace(regexp, '$.$.$') ); // 30.10.2019, 01.01.2020Non-capturing groups with ?:
Sometimes we need parentheses to correctly apply a quantifier, but we don’t want their contents in results.
A group may be excluded by adding ?: in the beginning.
For instance, if we want to find (go)+ , but don’t want the parentheses contents ( go ) as a separate array item, we can write: (?:go)+ .
In the example below we only get the name John as a separate member of the match:
let str = "Gogogo John!"; // ?: excludes 'go' from capturing let regexp = /(?:go)+ (\w+)/i; let result = str.match(regexp); alert( result[0] ); // Gogogo John (full match) alert( result[1] ); // John alert( result.length ); // 2 (no more items in the array)Summary
Parentheses group together a part of the regular expression, so that the quantifier applies to it as a whole.
Parentheses groups are numbered left-to-right, and can optionally be named with (?. ) .
The content, matched by a group, can be obtained in the results:
- The method str.match returns capturing groups only without flag g .
- The method str.matchAll always returns capturing groups.
If the parentheses have no name, then their contents is available in the match array by its number. Named parentheses are also available in the property groups .
We can also use parentheses contents in the replacement string in str.replace : by the number $n or the name $ .
A group may be excluded from numbering by adding ?: in its start. That’s used when we need to apply a quantifier to the whole group, but don’t want it as a separate item in the results array. We also can’t reference such parentheses in the replacement string.
Tasks
Check MAC-address
MAC-address of a network interface consists of 6 two-digit hex numbers separated by a colon.
For instance: '01:32:54:67:89:AB' .
Write a regexp that checks whether a string is MAC-address.
let regexp = /your regexp/; alert( regexp.test('01:32:54:67:89:AB') ); // true alert( regexp.test('0132546789AB') ); // false (no colons) alert( regexp.test('01:32:54:67:89') ); // false (5 numbers, must be 6) alert( regexp.test('01:32:54:67:89:ZZ') ) // false (ZZ at the end)A two-digit hex number is [0-9a-f] (assuming the flag i is set).
We need that number NN , and then :NN repeated 5 times (more numbers);
Now let’s show that the match should capture all the text: start at the beginning and end at the end. That’s done by wrapping the pattern in ^. $ .
let regexp = /^[0-9a-f](:[0-9a-f])$/i; alert( regexp.test('01:32:54:67:89:AB') ); // true alert( regexp.test('0132546789AB') ); // false (no colons) alert( regexp.test('01:32:54:67:89') ); // false (5 numbers, need 6) alert( regexp.test('01:32:54:67:89:ZZ') ) // false (ZZ in the end)