Detailed description of grouping and reverse reference of JavaScript regular expressions, and reverse reference of Regular Expressions

Detailed description of grouping and reverse reference of JavaScript regular expressions, and reverse reference of Regular Expressions

Syntax

Metacharacters (pattern): used for group matching repeatedly

Attribute $1 ~ $9 if it exists, it is used to obtain the matched substring in the corresponding group.

\ 1 or $1 is used to match the content in the first group.

\ 2 or $2 is used to match the content in the first group.

...

\ 9 or $9 is used to match the content in the first group.

Usage example

Var reg =/(A +) (B | C | D) +) (E +)/gi; // This regular expression has four groups // ing relations // RegExp. $1 <-> (A +) // RegExp. $2 <-> (B | C | D) +) // RegExp. $3 <-> (B | C | D) // RegExp. $4 <-> (E +)

The above code also provides $1 ~ $9 usage

$1 ~ $9 is a pre-defined static attribute of a regular expression, which is referenced by RegExp. $1.

Group nesting relationship description

The code above can also describe the nesting relationship of groups.

// Test environment: Chrome browser var str = "ABCDE"; var reg =/(A +) (B | C | D) +) (E +)/gi; str. match (reg); // output: ["ABCDE" reg.exe c (str, 'I'); // output: ["ABCDE", "A", "BCD ", "D", "E"] RegExp. $1; // output: "A" RegExp. $2; // output: "BCD" RegExp. $3; // output: "D" RegExp. $4; // output: "E"

In this way, we can clearly see the nested relationship of the group.

To sum up, when a large group contains a small group, a small group is the group that is placed after the large group, and so on.

Part 2

This section describes the usage of something similar to "\ 1 ".

Reverse reference of group matching

Syntax

Metacharacters \ 1 ~ \ 9: used to indicate a reference to a matched character or group.

Usage example

Reference: [original] AS3 js Regular Expression reverse reference)

There may be some interfaces mentioned above. Here is an example:

// In general, when we want to match any two identical characters (complicated: two identical groups), we can often use the following code: // description: // (\ w) is used to match any character except line breaks and tabs, while \ 1 is a reference to (\ w), so you can understand it as: (\ w) \ 1 is (\ w) // However, the difference between // (\ w) \ 1 and (\ w) is that, (\ w) indicates any two consecutive characters, such as Ac, MM, and K9. // But (\ w) \ 1 can only be AA or CC, 99 is A continuous and identical character // so you can understand that \ 1 is an instantiation reference to (\ w). When (\ w) matches, \ 1 is achieved by Table A. When (\ w) matches 9, \ 1 is expressed as 9. // if so much is said, it may be A bit nonsense, the following example shows how to understand var str = "AA Am 99"; var reg =/(\ w) \ 1/g; str. match (reg); // output: ["AA", "99"]

Therefore, refer to the example of "Regular Expression usage of highlighted keyword search" in the article I referenced above, and I will give a small DEMO of my own improvement.

Although this DEMO does not use any knowledge about reverse references: >_< ::

// Test environment: Chrome browser var key = "keywords"; // search keyword var text = "I am a text, and I have a keywords before this "; // The text to be matched var reg = new RegExp ("(" + key + ")", "g"); text. replace (reg, "<span style = 'color: red'> $1 </span>"); // output: "I am a text, and I have a <span style = 'color: red'> keywords </span> before this"

The following describes reverse references of Regular Expressions in detail.

Example 1:

public static void main(String[] args) {  String s="99-3933";  boolean b=Pattern.matches("([\\d])\\1[-]([3])\\1\\2{2}", s);  System.out.println(b); }

Reverse reference, matching repeated numbers

([\ D]) ===> \ 1

([3]) ==>\2

Example 2:

public class test {  public static void main(String[] args) {   String s="99-393399-3933";   boolean b=Pattern.matches("(([\\d])\\2[-]([3])\\2\\3{2})\\1", s);   System.out.println(b);  } }

Articles you may be interested in:

• Example of reverse expression reference in javascript
• AS3 js Regular Expression backreference)