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This article introduces the knowledge of "how to use regular expression RegExp". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Numbered capture groups

We know that regular expressions can be grouped, grouped in parentheses, and if you want to get the value of the grouping, it's called capture groups.

Generally speaking, we access capture groups by serial number, which is called Numbered capture groups.

For example:

Const RE_DATE = / ([0-9] {4})-([0-9] {2})-([0-9] {2}) /; const matchObj = RE_DATE.exec ('1999-12-31'); const year = matchObj [1]; / / 1999const month = matchObj [2]; / / 12const day = matchObj [3]; / / 31

The above regular expression matches the year, month and day, and then returns an array of match through the exec method. This array stores matching groups information.

Because we have three parentheses, we can match three group. Then access the specific group through 1Band 2Pol 3.

Let's take a look at the above matchObj output:

['1999-12-31, 1999, 12-31, index: 0, input:' 1999-12-31, groups: undefined]

You can see that matchObj is an array, and index 0 stores the string to match. Here we see that another groups of matchObj is undefined, and this groups is named groups.

Named capture groups

It is mentioned above that numbered capture groups accesses matching data through serial numbers. But the matching group does not have a name.

Let's take a look at how we can name these groups:

Const RE_DATE = / (? [0-9] {4})-(? [0-9] {2})-(? [0-9] {2}) /; const matchObj = RE_DATE.exec ('1999-12-31'); const year = matchObj.groups.year; / / 1999const month = matchObj.groups.month; / / 12const day = matchObj.groups.day; / / 31

Take a look at matchObj's content:

['1999-12-31,' 1999,'12,'31, index: 0, input: '1999-12-31, groups: [Object: null prototype] {year:' 1999, month:'12, day:'31}]

As you can see, this time there is more information about groups.

If you want to match the group information we matched before, you can use numbered groups\ k or named groups\ k.

Let's look at an example:

Const RE_TWICE = / ^ (? [Amurz] +)!! / trueRE_TWICE.test ('abcroomabc'); / / trueRE_TWICE.test (' abcroomab'); / / falseconst RE_TWICE = / ^ (? [Amurz] +)!\ 1 $/; RE_TWICE.test ('abcroomabc'); / / trueRE_TWICE.test (' abcroomab'); / / false

Both syntax can be used.

Named capture groups can also be used with replace.

With group name, we can use group name as a reference directly in replace:

Const RE_DATE = / (? [0-9] {4})-(? [0-9] {2})-(? [0-9] {2}) /; console.log ('1999-12-31'.replace (RE_DATE,'//$'))

The second parameter of replace can also be a function, and the parameter of the function is some of the contents of our group:

Const RE_DATE = / / (? [0-9] {4})-(? [0-9] {2})-(? [0-9] {2}) /; console.log ('1999-12-31'.replace (RE_DATE, (g0pime, day)) = > / / (A) month+'/'+day+'/'+year)); / 12 and 311999

In the above example, G0 = 1999-12-31 represents a matching substring. Y, m, d match numbered groups 1, 2, 2, 3.

Input is the entire input. {year, month, day} matches named groups.

Escape of Unicode attribute in RegExp

In the Unicode standard, every character has an attribute. To put it simply, attributes are used to describe this character.

For example, General_Category represents the classification of characters: X: General_Category = Lowercase_Letter

White_Space represents spaces, tabs and newline:\ t: White_Space = True

Age indicates when the character is added to the Unicode, and so on.

These properties also have corresponding abbreviations: Lowercase_Letter = Ll, Currency_Symbol = Sc, and so on.

For example, let's say we want to match spaces. The traditional practice is to do this:

> / ^\ s+$/.test ('\ t\ n\ r') true

Before that is the regular expression, then a test method is used to match the string, resulting in the true.

We just talked about the properties of unicode, and we can also match them with attributes:

> / ^\ p {White_Space} + $/ u.test ('\ t\ n\ r') true

Property matching uses\ p, followed by property values.

Note that we also add u after the regular expression to indicate that we are using the Unicode attribute escape.

Lookaround assertion

Lookaround assertion can be translated as a look assertion, which is a structure in regular expressions that is used to determine what the context of the object to match is like.

There are two kinds of lookaround assertion, one is Lookahead and the other is Lookbehind.

Let's take a look at the use of Lookahead:

Const RE_AS_BS = / aa (? = bb) /; const match2 = RE_AS_BS.exec ('aabb'); console.log (match2 [0]); / /' aa'const match3 = RE_AS_BS.exec ('aab'); console.log (match3); / / null

Lookahead is looking forward, and above we use (? = bb) to match the bb forward.

Note that although the regular expression matches with aabb, bb is not included in match.

The result is that the first one matches, and the second one doesn't match.

Besides using? =, we can also use?! Means unequal:

> const RE_AS_NO_BS = / aa (?! bb) /; > RE_AS_NO_BS.test ('aabb') false > RE_AS_NO_BS.test (' aab') true > RE_AS_NO_BS.test ('aac') true

Let's look at the use of Lookbehind again.

Lookbehind and Lookahead queries go in exactly the opposite direction.

Backward matching is done using? const RE_NO_DOLLAR_PREFIX = / (? dotAll flag

Under normal circumstances, dot. Represents a character, but this character cannot represent the Terminator of a line:

> / ^ .$ / .test ('\ n') false

And dotAll is in dot. Match the s introduced later, which can be used to match the Terminator of a line:

> / ^. $/ s.test ('\ n') true

In ES, the following characters represent the Terminator of a line:

Utt000A LINE FEED (LF) (\ n)

Ubun000D CARRIAGE RETURN (CR) (\ r)

Ugg 2028 LINE SEPARATOR

Ugg 2029 PARAGRAPH SEPARATOR

That's all for "how to use regular expressions RegExp". Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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