PHP中的正则表达式_PHP
正则表达式
综述正则表达式是一种可以用于模式匹配和替换的强有力的工具。正则表达式可以让用户通过使用一系列的特殊字符构建匹配模式，然后把匹配模式与数据文件、程序输入以及WEB页面的表单输入等目标对象进行比较，根据比较对象中是否包含匹配模式，执行相应的程序。
如何使用基本模式匹配?
模式，是正规表达式最基本的元素，它们是一组描述字符串特征的字符。模式可以很简单，由普通的字符串组成，也可以非常复杂，往往用特殊的字符表示一个范围内的字符重复出现，或表示上下文。我们先看一些正则表达式中的特殊字符。
特殊字符"^"用来匹配以指定字符串开头的字符串。例如：
"^hello" ：这个模式与字符串"hello，PHP world!"匹配，但是与"Say hello to you"不匹配。
特殊字符"$"用来匹配以指定字符串结尾的字符串。例如：
"you$" ：这个模式与"How are you"匹配，与"your"不匹配。
当特殊字符"^"和"$"同时使用时，表示精确匹配。例如：
"^hello$" ：这个模式只匹配字符串"hello"。
如果一个模式不包括"^"和"$"，那么它与任何包含该模式的字符串匹配。例如： "you" ：与字符串"What is your name?"是匹配的。
在该模式中的字母只是普通的字符，数字也是一样的。
如果要用到其他一些稍微复杂的字符，如标点符号和空白字符（比如空格、制表符等），就要要用到转义序列。所有的转义序列都用反斜杠("\")打头，例如制表符的转义序列是："\t"。所以如果我们要检测一个字符串是否以制表符开头，可以用这个模式：
"^\t"
类似的，用"\n"表示换行，"\r"表示回车，反斜杠本身用"\\"表示，句号"."用"\."表示，依此类推。
如何使用字符簇?
如果要判断用户输入的电话号码、地址、EMAIL地址、信用卡号码等是否有效，用普通的基于字面的字符串比较是不够的。所以要用一种更好的方法来描述我们想要的模式，这就是字符簇。
比如，要建立一个表示所有元音字符的字符簇，就可以这样做：
"[AaEeIiOoUu]" ：这个模式与任何元音字符匹配，但只能表示一个字符。
用特殊符号"-"可以表示一个字符的范围，如：
| "[a-z]" | //匹配字母a-z，即所有的小写字母 |
| "[A-Z]" | //匹配字母A-Z，即所有的大写字母 |
| "[a-zA-Z]" | //匹配所有的字母 |
| "[0-9]" | //匹配所有的数字 |
| "[0-9\.\-]" | //匹配所有的数字，以及句号和减号 |
| "[ \f\r\t\n]" | //匹配所有的白字符 |
同样的，这些也只匹配一个字符。
如果要匹配由一个小写字母和一位数字组成的字符串，比如"a4"、"b5"或"f1"，但不是"aa4"、"b5a4" 或"f12"的话，用这个模式：
"^[a-z][0-9]$"
尽管[a-z]代表26个字母的范围，但在这里它只能与第一个字符是小写字母的字符串匹配。
我们已经知道"^"表示字符串的开头，但是当在一组方括号里使用"^"时，它表示"非"或"排除"的意思，常常用来剔除某个字符。还用前面的例子，我们要求第一个字符不能是数字： "^[^0-9][0-9]$"
这个模式与"a4"、"b5"及" 2"是匹配的，但与"12"、"66"是不匹配的。下面是几个排除特定字符的例子：
"[^a-z]" //除了小写字母以外的所有字符
"[^\\\/\^]" //除了(\)(/)(^)之外的所有字符
"[^\"\']" //除了双引号(")和单引号(')之外的所有字符
特殊字符"." (点，英文句号)在正规表达式中用来匹配除了"换行"之外的所有字符。所以模式"^.5$"与任何两个字符的、以数字5结尾和以其他非"换行"字符开头的字符串匹配。模式"."可以匹配任何字符串，除了空串和只包括一个"换行"的字符串。
PHP的正规表达式有一些内置的通用字符簇，列表如下：
|
字符簇
|
含义
|
| "[[:alpha:]]" | 任何字母 |
| "[[:digit:]]" | 任何数字 |
| "[[:alnum:]]" | 任何字母和数字 |
| "[[:space:]]" | 任何白字符 |
| "[[:upper:]]" | 任何大写字母 |
| "[[:lower:]]" | 任何小写字母 |
| "[[:punct:]]" | 任何标点符号 |
| "[[:xdigit:]]" | 任何16进制的数字，相当于[0-9a-fA-F] |
在很多的情况下，我们可能要匹配一个单词或一组数字。一个单词有若干个字母组成，一组数字有若干个单一的数字组成。我们用跟在字符或字符簇后面的花括?quot;{}"来确定前面的内容的重复出现的次数：假设x是一个数字，那么{x}表示"前面的字符或字符簇只出现x次"；一个数字加逗号，{x，}的意思是"前面的内容出现x或更多的次数"；两个用逗号分隔的数字，{x，y}表示"前面的内容至少出现x次，但不超过y次"。
|
字符簇
|
含义
|
| "^[a-zA-Z_]$" | 所有的字母和下划线 |
| "^[[:alpha:]]{3}$" | 所有的3个字母的单词 |
| "^a$" | 字母a |
| "^a{4}$" | 不是以字母a开头并且有4个字母的单词，比如Aaaa |
| ^a{2，4}$" | aa，aaa或aaaa |
| "^a{1，3}$" | a，aa或aaa |
| "^a{2，}$" | 包含多于两个a的字符串，比如aaa，aaaa，aaaaa |
| "^a{2，}" | 以两个a开头的单词，如：aardvark和aaab，但apple不行 |
| "a{2，}" | 包含有两个a的单词，如：baad和aaa，但Nantucket不行 |
| "\t{2}" | 两个制表符 |
| ".{2}" | 所有的两个字符 |
我们可以把模式扩展到更多的单词或数字：
| "^[a-zA-Z0-9_]{1，}$" | 所有包含一个以上的字母、数字或下划线的字符串 |
| "^[0-9]{1，}$" | 所有的正数 |
| "^\-{0，1}[0-9]{1，}$" | 所有的整数 |
| "^\-{0，1}[0-9]{0，}\.{0，1}[0-9]{0，}$" | 所有的整数 |
最后一个例子我们可以这样考虑：所有以一个可选的负号(\-{0，1})开头(^)、跟着0个或更多的数字([0-9]{0，})、和一个可选的小数点(\.{0，1})再跟上0个或多个数字([0-9]{0，})，并且没有其他任何东西($)。
特殊字符"?"与"{0，1}"是相等的，它们都代表着："0个或1个前面的内容"或"前面的内容是可选的"。所以：
"^\-{0，1}[0-9]{0，}\.{0，1}[0-9]{0，}$"
可以简化为：
^\-?[0-9]{0，}\.?[0-9]{0，}$
特殊字符"*"与"{0，}"是相等的，它们都代表着"0个或多个前面的内容"。字符" "与{1，}是相等的，表示"1个或多个前面的内容"，所以上面的4个例子可以写成：
| "^[a-zA-Z0-9_] $" | 所有包含一个以上的字母、数字或下划线的字符串 |
| "^[0-9] $" | 所有的正数 |
| "^\-?[0-9] $" | 所有的整数 |
| "^\-?[0-9]*\.?[0-9]*$" | 所有的小数 |
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