How JavaScript uses regular expressions

Regular expression is a pattern used to match strings and can be used to verify, search, replace strings, etc. JavaScript has built-in support for regular expressions. This article will introduce how to use regular expressions in JavaScript.

Regular expressions are complex and may be confusing if you are new to them. But as long as you understand the basic syntax and common functions, you can use it to solve complex string matching and replacement problems.

1. Basic syntax of regular expressions

Regular expressions consist of a series of characters and metacharacters, and metacharacters are used to specify matching rules. The following are some of the most basic metacharacters:

1. Letters and numbers: You can enter all letters and numbers directly in the regular expression to match the corresponding characters. For example, the regular expression /test/ can match the string "test" exactly.
2. Metacharacters: Metacharacters are special characters with special meanings. For example, "." in the regular expression represents any character, "^" represents the beginning of the string, and "$" represents the end of the string.
3. Character set: used to match any character in a set of characters. For example, the regular expression /[ab]/ matches "a" or "b".
4. Quantifier: used to specify the number or range of matching repetitions. For example, the regular expression /a / can match any number of "a" characters (at least one), and "a{3}" means matching three consecutive "a" characters.

Here are some simple regular expression examples:

Expression Matching content
/test/ test
/.at/ cat, hat, sat
/[bc]at/ bat, cat
/[0-9]*/ 1234567890

2. Regular expressions in JavaScript

JavaScript has built-in support for regular expressions. Use the RegExp object to create a regular expression object, or you can use the regular method of the String object to perform string matching.

1. Create a regular expression object

You can use the constructor of the RegExp object to create a regular expression object. The constructor accepts two parameters: one is for matching string mode, the other is an optional flag string.

For example:

var pattern = /\d /g; //Use literal pattern to create a regular expression object
var pattern2 = new RegExp("\d ", "g "); //Use the constructor to create a regular expression object

In the above example, the regular expression "/\d /g" and the string "\d " are the same, both indicating matching or multiple numbers.

2. String matching

In JavaScript, you can use the match(), search(), replace(), and split() methods of the String object to implement string matching.

match() method: The match() method returns an array that contains the string content that matches the regular expression. If nothing is matched, null is returned. For example:

var str = "hello world, hello JavaScript";
var pattern = /hello/;
var result = str.match(pattern);
console.log(result); // Output[ "hello"]

search() method: The search() method returns the starting position of the matching string. If nothing is matched, it returns -1. For example:

var str = "hello world, hello JavaScript";
var pattern = /world/;
var result = str.search(pattern);
console.log(result); // Output 6

replace() method: The replace() method is used to replace the matched text content and return the replaced new string. For example:

var str = "hello world, hello JavaScript";
var pattern = /hello/;
var result = str.replace(pattern, "hi");
console .log(result); // Output "hi world, hi JavaScript"

split() method: The split() method splits a string into a string array, using a specified delimiter string Or regular expression to determine the split position. For example:

var str = "apple,banana,orange";
var pattern = /[,]/;
var result = str.split(pattern);
console.log (result); // Output ["apple", "banana", "orange"]

3. Verification string

In JavaScript, you can use the RegExp object The test() method checks whether the string conforms to the regular expression. test() returns true or false, indicating whether the string meets the regular expression requirements. For example:

var pattern = /\d /;
console.log(pattern.test("123")); // Output true
console.log(pattern.test("1a23 ")); // Output true

3. Advanced applications of regular expressions

There are many advanced uses of regular expressions, which can achieve more complex string matching and replacement functions. Here are some common advanced applications:

1. Group matching

Use parentheses to indicate grouping, for example, the regular expression /(\d{3})-( \d{3})-(\d{4})/ can match phone number formats like "123-456-7890", where (\d{3}), (\d{3}) and (\ d{4}) These three groups match the area code, center number, and suffix number in the phone number respectively.

2. Zero-width assertion

Zero-width assertion refers to matching the position of the string rather than the character. For example, the regular expression /cat(?=dog)/ matches "cat" followed by "dog", but not "cat" followed by "dog".

3. Replace matching results

We can use placeholders such as $1, $2, etc. when replacing text to represent the first and second groups in the matching results. And so on. For example, the regular expressions /(\d{3})-(\d{3})-(\d{4})/ and "123-456-7890" can be replaced with "($1) $2-$3" , the result is "(123) 456-7890".

4. Conclusion

Regular expression is a powerful pattern matching tool. Learn to use it to handle various string matching and replacement problems. In JavaScript, you can use the regular methods of the RegExp object and the String object to perform regular expression operations. This article introduces the basic syntax and common applications of regular expressions, and provides some examples of advanced applications. I hope readers can understand the use of regular expressions in JavaScript through this article.

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