JavaScript Regular Expressions Browser Differences

Regular expressions in JavaScript may produce different results in different browsers. Below, the results of regular expressions in the five major browsers (IE, Firefox, Chrome, Safari, Opera, subject to the current version) Let’s sort out and list the differences.

1. Firefox and Chrome will over-optimize regular expressions created within loops (and function definitions embedded within them), seemingly assuming that people writing JavaScript will write regular expression construction and assignment in the wrong place.

var r;
for(var i = 0; i < 2; i++){
  var x = /abc/g;
  if(r)
    // 第二个循环时Firefox和Chrome会输出“true”
    document.write(r == x); 
  else
    r = x;
}

2. If you pass a function that may return nothing as the second parameter to the replace method, then IE may directly delete the matching text (in the previous test I summarized the behavior comparison of Opera unique, which now seems to be a wrong conclusion), while other browsers will replace the matching text with "undefined".

// IE会输出“13”，而其它浏览器输出“1undefined3”
document.write('123'.replace(/2/, function(){}));

3. If you use an existing regular expression instance as a parameter when creating a regular expression in the form of new RegExp, most browsers will create a new regular expression with the same basic functions but completely independent instance; Safari will simply return the regular expression instance as the parameter.

var r = /1/;
// Safari会输出“true”，而其它浏览器输出为“false”
document.write(new RegExp(r) == r);

4. If you convert an "empty" regular expression directly into a string, browsers other than IE will get "/(?:)/", and IE will get "//" - but from the regular expression When the source attribute is directly extracted from the expression, all the results are empty strings.

// IE输出“//”，其它浏览器输出“/(?:)/”
document.write(new RegExp('')); 
// IE输出“undefined”，其它浏览器输出“/(?:)/”
document.write(eval('' + new RegExp('')))

5. If you convert a regular expression containing a slash "/" directly into a string - take "new RegExp('/')" as an example, only Firefox and Opera will get "///", And directly extracting the source attribute will get "/"; while other browsers will get "///", and directly extracting the source attribute will get "/".

// Firefox和Opera输出“/\//”，其它浏览器输出“///”
document.write(new RegExp('/')); 
// Firefox和Opera输出“/\//”，其它浏览器输出“undefined”
document.write(eval('' + new RegExp('/')))

6. If an invalid option flag (such as "/abc/n") is used when using a literal expression to define a regular expression, Chrome and Safari will completely ignore the invalid option flag (equivalent to " /abc/"), which will cause syntax errors in other browsers.

// Chrome和Safari会输出“/abc/”，其它浏览器中产生语法错误
document.write(/abc/n);

7. If when creating a regular expression through the constructor, the (second) string parameter specifying the option flag contains characters that are not valid flags, an exception will be caused in Firefox, and otherwise The browser will ignore the invalid part.

// Firefox中会引起一个异常，提示无效的标志；而其它浏览器会输出“/1/
document.write(new RegExp('1', 'n')); ”

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