Details about how to judge the code of various browsers using JS Regular Expressions

All annotations are in Code . Very detailed.
I only judged ie Firefox Google because I didn't install any other browsers, so haha. Of course, if you want to judge other browsers. The basic code is the same. Copy code The Code is as follows: (function (){
Window. sys = {};
VaR UA = navigator. useragent. tolowercase ();
// First, let's take a look at what the three browsers return.
// Ie UA = Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; qqpinyin 730;. Net CLR 1.1.4322)
// Firefox UA = Mozilla/5.0 (windows; U; Windows NT 5.2; ZH-CN; RV: 1.9.2) Gecko/20100115 Firefox/3.6
// Chrome UA = Mozilla/5.0 (windows; U; Windows NT 5.2; en-US) applewebkit/532.5 (khtml, like gecko) Chrome/4.0.249.0 Safari/532.5

// You can see the preceding Browser ie msie 8.0 Firefox/3.6 Chrome/4.0.249
// We only need to use regular expressions to find out which browser can match the key points in the string.
// See the following regular Firefox Regular Expression/Firefox \/([\ D \.] + )/
// Firefox \/matches a character like Firefox/
// ([\ D \.] +) () indicates the character to be captured. It is his credit to use [1] to obtain the version number.
// [] Indicates that \ D \. is included as a whole.
// \ D match a number \. Match A. Add together to match 8. Something like this
// [] + [] There is a + outside, which will repeat the originally matched items in [] and turn it into this 8.8.8.8. Of course, this can also be 48.1.563
SYS. Firefox = UA. Match (/Firefox \/([\ D \.] + )/);
SYS. Ie = UA. Match (/MSIE \ s ([\ D \.] + )/);
// MSIE \ s ([\ D \.] +)
// IE is basically the same as Firefox.
// MSIE 8.0 \ s is a space matching between MSIE and 8.0
SYS. Chrome = UA. Match (/chrome \/([\ D \.] + )/);
// Chrome \/([\ D \.] +)
// Same as Firefox
})();

// Use
If (SYS. Firefox ){
Alert (true)
} Else {
Alert (false)
}
If (SYS. ie [1] = "8.0 "){
Alert (true)
} Else {
Alert (false)
}

Alert (SYS. Firefox );
Alert (SYS. ie );
Alert (SYS. Chrome );
// Only the current browser's sys can use [1] to return the version number
Alert (SYS. Firefox [1]);
// Alert (SYS. ie [1]);
// Alert (SYS. Chrome [1]);

I encountered a problem. I can see that the experts wrote this.

Firefox \/([\ D.] +) // expert
Firefox \/([\ D \.] +) // my
// I am confused about the differences between the two. I used a lot of STR tests. I didn't see any difference. // is [\ D.] + the same as [\ D \.] +? Please give answers.