String connection

+/+= Operator join

str += "one" + "two";

This is a common way to connect strings , It runs through the following four steps :

1、 Create a temporary string in memory ;

2、 Connected ”onetwo” Assigned to this temporary string ;

3、 Temporary string and str The current value of the connection ;

4、 The result of the connection is assigned to str.

The downstream code can avoid producing temporary strings

str = str + "one" + "two";

The assignment expression is given by str Start as a foundation , Attach it one string at a time , Connect from left to right , So you avoid using temporary strings . If you change the order such as hold str Put it in the middle Then there is no optimization effect . This is related to the way the browser allocates memory when merging strings .

Array item merging

Array.prototype.join Method to combine all the elements of an array into a single string , Receives a parameter as a connector .

var str = "i am people but i very shuai a",
Newstr = "",
Appends = 5000; while(Appends --){
Newstr += str

This code connects 5000 A length of 30 String , The following figure for IE7 The time it takes to complete

IE7 5000 The first merger used 226 Milliseconds have a significant impact on performance , How do you optimize it ?

Use array item merging to generate a generic string

var str = "i am people but i very shuai a",
Strs = [],
Newstr = "",
Appends = 5000; while(Appends --){
Strs[Strs.length] = str;
Newstr = Strs.join("");

IE7 test result

By avoiding the repeated allocation of memory and copying the increasing string , So the performance improvement is obvious .


Concat Can receive any number of parameters , And attach each parameter to the called string .

str = str.concat(s1);

str = str.concat(s1,s2,s3);

Unfortunately , Use concat It's easier than using + and += Slightly slower , Especially in IE opera and Chrome The slower is more obvious .

Regular expression optimization

How regular expressions work , Understanding the principle helps to better solve all kinds of problems that affect the performance of regularization .

compile : Browsers validate regular expression objects , Then turn it into a native code program ; Assign a regular object to a variable , It can avoid repeated compilation .

Set start position : The starting search position of the target string , It's usually the starting character of a string 、 Or regular lastIndex Property to specify the location ( Limited to with /g Of exec and test)、 Or the next character of the last matching character when returning from the fourth step .

The way browsers optimize regular expression engines is , In this stage, we skip some unnecessary work through early prediction . for example , If a regular expression takes ^ start ,IE and Chrome Usually judge whether it can match at the beginning of the string , Then you can avoid foolishly searching for subsequent locations . Another example is matching. The third letter is x String , A smart way is to find out first x, And then it goes back two characters from the starting position .

Match each regular expression character : Start at the beginning of the string , Check the text and regular patterns one by one , When a particular character fails to match , Go back to where you tried to match , Try other possible paths .

Match success or failure : If there is an exact match in the current string position , The match is successful ; If the current location doesn't have all the possible paths, it doesn't match successfully , Will go back to the second step , Reset the starting position , Start another round of matching … Until the last string is the actual position , Not yet , The match fails .

Understanding backtracking (Backtracking)

Backtracking is an essential part of the matching process , That's why regularity is so powerful , It's also the performance cost of regularization , So how to reduce backtracking is the key to improve regularity . Backtracking usually occurs in the case of branching and repetition :

Branching and backtracking

/h(ello|appy) hippo/.test("hello there, happy hippo");

It starts with h And the starting position of the string h matching , The next branch , From left to right ,(ello|appy) Medium ello First try to match , character string h Back there, too ello, The match is successful , So we continue to match (ello|appy) Space after , Still match successfully , Continue to match after the space in the regular h, The string space is followed by t, Matching failure .

Back to the regular Branch (ello|appy)( This is backtracking ), Try to use appy For the first character of a string h After the character matching , Failure , There are no more options , No more backtracking .

The first start position match failed , The starting position is one bit later , Rematch h… Until the string starts at 14 when , Match to h.

So we started a new round of character matching , Enter the branch (ello|appy) Medium ello, Matching failure .

Back to the regular Branch (ello|appy)( Back again ),appy The match is successful , Exit the branch , Match the following hippo, Match string happy hippo, The match is successful , End match .

Repetition and backtracking

var str = "<p>Para 1.</p><img src='smiley.jpg'><p>Para 2.</p><div>Div.</div>";


It starts with <p> And the starting position of the string <p> matching , Next is .*(. Match any character other than the newline ,* It's a greedy quantifier , It means repetition 0 Times or times , Match as many times as possible ),.* Matches all the characters that follow up to the end of the string .

Try to match .* hinder <, Failed to match at the end of the string , Then backtrack one character at a time to try to match …, until </div> The first character of is matched successfully , Next, in regular \/ It's also related to / The match is successful , Continue to match... In regular p, Matching failure , return </div>, Keep going back , Until the second paragraph </p>, The match is successful , return <p>Para 1.</p><img src=’smiley.jpg’><p>Para 2.</p>, There are 2 A paragraph and a picture , End match .

Backtracking is out of control

When backtracking got out of control , May cause the browser to feign death for a few seconds 、 A few minutes or more , Let's take the following example ( To match the whole HTML character string ):


The matching structure is complete html When you file , Everything is all right , But when some tags are missing, the problem arises , If html At the end of the document </html> defect , the last one [\s\S]*? The repetition extends to the end of the string , matching </html> Failure , The regularizer searches forward in turn and remembers the backtracking position for subsequent use , When regular expressions extend to the penultimate [\s\S]*?—— Use it to match the <\/body> The matching one <body>,—— And then continue to look for </body> label , Until the end of the string . When all the steps fail , The third from the bottom [\s\S]*? Will be extended to the end of the string , And so on .

Back to the ultimate solution : Simulated atomic group ( Look ahead + backreferences ):(?=([\s\S]*?<head>))\1


Atomic group ( Look ahead ) Any backtracking position of is discarded , Avoid the backtracking out of control from the root , But looking forward doesn't consume any characters as part of the global match , Capture group + Back references can be used here to solve this problem , It's important to note that this is the number of back references , It's the one above \1、\2、\3、\4 Corresponding position .

No other details ( Because I can't understand ).

When not to use regular expressions

If it's just a search string , And know in advance which part of the string needs to be tested , Regularization is not the best solution . such as , Check whether a string ends with a semicolon :

/;$/.test(str); Regularization starts with the first character , Test the whole string one by one , See if she's a semicolon , At the end of the string , When the string is long , The more time it takes .

str.charAt(str.length – 1) == “;”; This jumps directly to the last character , Check if it's a semicolon , The string is very small, maybe just a little faster , But for long strings , Length does not affect the time required .

String native methods are fast , such as slice、substr、substring、indexOf、lastIndexOf etc. , They can avoid the performance overhead of regularization .

Remove the leading and trailing whitespace of the string

String.prototype.trim = function() {
var str = this.replace(/^\s+/, ""),
end = str.length - 1,
ws = /\s/;
while (ws.test(str.charAt(end))) {
return str.slice(0, end + 1);

This solution uses regularization to remove the white space in the head , Position anchor ^, It will be soon , It's mainly the blank processing of the tail , As mentioned above when not to use regular expressions , Regular is not the best , Here we use the string native method combined with regularization to solve , You can avoid performance being affected by the length of strings and whitespace .

Summary :

When connecting a large number or size of strings , Array item merging is unique in IE7 And earlier versions of reasonable performance methods .

Don't consider IE7 Words , Array item merging is one of the slowest ways to join strings . Recommended + and += Operator instead of .

Backtracking is an essential part of regular expression matching , It's also the low efficiency of regularization .

Runaway backtracking occurs where regularization is supposed to match quickly , However, some special string matching actions lead to slow running and even browser crash . The way to avoid this problem is : Make adjacent resources mutually exclusive , Avoid nested quantifiers matching the same part of the same string many times , Remove unnecessary backtracking by reusing the atomic set of the preview .

Regular expressions are not always the best tool to do the job , Especially when you only search for literal strings .

There are many ways to remove the ending whitespace of a string , But with two simple regular expressions ( One to remove the head, one to remove the tail ) To handle a lot of string content can provide a simple and cross browser method .

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