Definition and introduction of javascript regular expressions
JavaScript Regular Expression
Regular Expression (English: Regular Expression, often abbreviated as regex, regexp or RE in code) uses a single string to describe and match a series of matches A string search pattern for a certain syntax rule.
Search mode can be used for text search and text replacement.
What is a regular expression?
Regular expression is a search pattern formed by a sequence of characters.
When you search for data in text, you can use search patterns to describe what you want to query.
A regular expression can be a simple character, or a more complex pattern.
Regular expressions can be used for all text search and text replacement operations.
1. Define a regular expression through the RegExp constructor. The first parameter is the regular expression passed in as a string, and the second parameter is the modifier also passed in as a string.
例：var caps = new RegExp("[A-Z]", "g");2. Directly define a regular expression literal, separated by a forward slash symbol "/" on the right side of the expression, followed directly by the modifier. (This is recommended)
例：var caps = /[A-Z]/g;
Commonly used characters in javascript regular expressions
| Characters | Description | Example |
|---|---|---|
| [exp] | Regular expression handler for character sequences wrapped in square brackets ([]) Will match any character inside the square brackets. | [exp] Matches any character among e, x, p |
| [^exp] | Add ^ in front of the square brackets Characters, match to any symbol outside the square brackets, except the meaning . | [^exp] matches any character except e, x, p |
| [exp1-exp2] | Use - hyphen to express Matches any character in the character sequence from exp1 to exp2. | [A-Z] matches any character from A-Z; [0-9] matches any number from 0-9 |
| (exp) | For small A character sequence wrapped in brackets, the regular expression will match exactly in this character sequence (it will match if it contains) . | (exp) matches the string "exp" |
| (exp1\ | exp2) | Use the pipe character\ |
| exp+ | Use the + character after the expression to indicate that is matched only when the expression is included one or more times. | exp+match exp |
| match whitespace characters, that is, spaces, tabs, carriage returns | A\sB matches a string containing A whitespace character B | |
| matches any character except whitespace characters | A\SB matches A is not a whitespace character. B | |
| matches a number from 0-9 | ||
| Matches any character except numbers | ||
| Matches one A literal character, that is, a word, number or letter | ||
| matches any character except literal characters |
| Description | ||
|---|---|---|
| Apply regular The expression finds all matching items in the comparison string instead of just returning the first match | ||
| Apply the expression to compare and match at the same time , ignore case | ||
| Apply expressions to compare and match multiple lines of text, not just the first line |
| 字符序列 | 含义 | 例子 |
|---|---|---|
| $$ | 把找到的字符串用一个单独的$字符代替 | "Hello World".replace(/o/g, "\$\$");//"Hell\$ W\$rld" |
| $& | 使用第一个参数中所给定的字符串来替换所找到的子字符串 | "Hello World".replace(/o/g, "$&");//"Hello World" |
| $`(ESC下面那个`) | 使用所找到的字符串之前的文本来替换该子字符串 | "Hello World".replace(/o/g, "$`");//"HellHell WHello Wrld" |
| $' | 使用所找到的字符串之后的文本来替换该子字符串 | "Hello World".replace(/o/g, "$'");//"Hell World Wrldrld" |
| \$1, \$2 | 当第一个参数中包含的正则表达式使用小括号对进行表达式分组，则可以提取出特定的表达式所匹配的子字符串（\$1对应第一个小括号，\$2对应第二个小括号） | "Hello World".replace(/(o)(r)/g, "\$1\$2\$1\$2");//"Hello Wororldd"(查找到or，然后替换成oror |
还有一种就是replace() 的第二个参数可以用函数方式传入，然后用函数的返回值来替换原子字符串。
var count = 0;
function replaceWithCount() {
count = count + 1;
return count;
}
alert("Hello World".replace(/o/g, replaceWithCount)); //Hell1 W2rld
alert("Hello World".replace(/\s/g, replaceWithCount)); //Hello3World总结
这只是我写的作为笔记的正则表达式一些简单的判断和javascript用法。正则表达式还可以很复杂很强大，那就要再你要使用的时候再去挖掘思考了。如果想了解更多，可以学习Mozilla开发者网络上的关于javascript中使用正则表达式的指引介绍。写得更加详细。
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