JavaScript Regular expression-reverse reference

Use parentheses "()" to group, so that sub-expressions (sub-patterns) can be decorated as a whole, the results of sub-expressions matching are recorded and can be accessed separately.

/(A (b (CD) {2}) +) ef/

The references correspond to each other:

\1 Correspondence (A (b (CD) {2}) +)

\2 Correspondence (b (CD) {2})

\3 Correspondence (CD)

str = "Pic1.gif gif2.gif pic.gif";

Reg =/(. +) ([1-4]) \.\1/;

Arr_m = Str.match (reg);//arr_m = ["Gif2.gif", "gif", "2"]

Parse: (. +) Match gif

([1-4]) matches a number between 1~4

\. Immediately following "."

\1 referencing the first sub-expression

str = "numA2 numa2numb3c4c4 numd5e6e6numd5e6d5";

reg =/(num ([a-z][0-9]) {2}) \2+/g;

Arr_m = Str.match (reg);//arr_m = ["Numb3c4c4", "Numd5e6e6"];

str = "Happy is the Selection";

Reg =/(^\w+) \b (. *) \b (\w+$)/;

Arr_m = Str.match (reg);

STR2 = Str.replace (Reg,)

JavaScript Regular expression-reverse reference