PHP 正则表达式
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PHP正则表达式 正则表达式通常称为regex。这些只不过是一个模式或一系列字符，它们将特殊的搜索模式描述为t…
PHP正则表达式
正则表达式通常称为regex。这些只不过是一个模式或一系列字符，它们将特殊的搜索模式描述为text字符串。
正则表达式允许你搜索某个字符串在另一特定字符串。即使我们可以用另一个字符串替换一个字符串，也可以将一个字符串分成多个块。他们使用算术运算符(+，-，^)创建复杂的表达式。
默认情况下，正则表达式区分大小写。
正则表达式的优点和用途
在当前的应用程序编程中，几乎所有地方都使用正则表达式。下面给出了正则表达式的一些优点和用法：
- 正则表达式可帮助程序员验证文本字符串。
- 它提供了功能强大的工具来分析和搜索模式以及修改文本字符串。
- 通过使用正则表达式功能，可以提供简单的解决方案来识别模式。
- 正则表达式有助于创建识别标签的HTML模板系统。
- 正则表达式广泛用于浏览器检测，表单验证，垃圾邮件过滤和密码强度检查。
- 它对用户输入验证测试(例如电子邮件地址，手机号码和IP地址)很有帮助。
- 它有助于根据搜索结果或输入突出显示文件中的特殊关键字。
- 元字符使我们可以创建更复杂的模式。
您可以通过应用一些正则表达式的基本规则来创建复杂的搜索模式。正则表达式还使用许多算术运算符(+，-，^)创建复杂的模式。
正则表达式运算符
| Operator | Description |
|---|---|
| ^ | It indicates the start of string. |
| $ | It indicates the end of the string. |
| . | It donates any single character. |
| () | It shows a group of expressions. |
| [] | It finds a range of characters, e.g., [abc] means a, b, or c. |
| [^] | It finds the characters which are not in range, e.g., [^xyz] means NOT x, y, or z. |
| – | It finds the range between the elements, e.g., [a-z] means a through z. |
| | | It is a logical OR operator, which is used between the elements. E.g., a|b, which means either a OR b. |
| ? | It indicates zero or one of preceding character or element range. |
| * | It indicates zero or more of preceding character or element range. |
| + | It indicates zero or more of preceding character or element range. |
| {n} | It denotes at least n times of preceding character range. For example – n{3} |
| {n, } | It denotes at least n, but it should not be more than m times, e.g., n{2,5} means 2 to 5 of n. |
| {n, m} | It indicates at least n, but it should not be more than m times. For example – n{3,6} means 3 to 6 of n. |
| It denotes the escape character. |
正则表达式中的特殊字符类
| Special Character | Description |
|---|---|
| n | It indicates a new line. |
| r | It indicates a carriage return. |
| t | It represents a tab. |
| v | It represents a vertical tab. |
| f | It represents a form feed. |
| xxx | It represents an octal character. |
| xxh | It denotes hexadecimal character hh. |
PHP提供了两组正则表达式函数：
- POSIX正则表达式
- PERL样式正则表达式
POSIX正则表达式
POSIX正则表达式的结构类似于典型的算术表达式：几个运算符/元素组合在一起以形成更复杂的表达式。
最简单的正则表达式是匹配字符串单个字符的表达式。例如-在切换或笼形字符串内的“g”。让我们介绍一下POSIX正则表达式中使用的一些概念：
括号
在正则表达式中使用方括号[]具有特殊含义。这些用于查找其中的字符范围。
| Expression | Description |
|---|---|
| [0-9] | It matches any decimal digit 0 to 9. |
| [a-z] | It matches any lowercase character from a to z. |
| [A-Z] | It matches any uppercase character from A to Z. |
| [a-Z] | It matches any character from lowercase a to uppercase Z. |
以上范围是常用的。您可以根据需要使用范围值，例如[0-6]以匹配0到6之间的任何十进制数字。
量词
特殊字符可以表示方括号和单个字符的位置。每个特殊字符都有特定的含义。给定的符号+，*，？，$和{intrange}标志均遵循字符序列。
| Expression | Description |
|---|---|
| p+ | It matches any string that contains atleast one p. |
| p* | It matches any string that contains one or more p’s. |
| p? | It matches any string that has zero or one p’s. |
| p{N} | It matches any string that has a sequence of N p’s. |
| p{2,3} | It matches any string that has a sequence of two or three p’s. |
| p{2, } | It matches any string that contains atleast two p’s. |
| p$ | It matches any string that contains p at the end of it. |
| ^p | It matches any string that has p at the start of it. |
PHP Regexp POSIX函数
PHP提供了七个使用POSIX样式正则表达式搜索字符串的函数-
| Function | Description |
|---|---|
| ereg() | It searches a string pattern inside another string and returns true if the pattern matches otherwise return false. |
| ereg_replace() | It searches a string pattern inside the other string and replaces the matching text with the replacement string. |
| eregi() | It searches for a pattern inside the other string and returns the length of matched string if found otherwise returns false. It is a case insensitive function. |
| eregi_replace() | This function works same as ereg_replace() function. The only difference is that the search for pattern of this function is case insensitive. |
| split() | The split() function divide the string into array. |
| spliti() | It is similar to split() function as it also divides a string into array by regular expression. |
| Sql_regcase() | It creates a regular expression for case insensitive match and returns a valid regular expression that will match string. |
注意：请注意，以上功能在PHP 5.3.0中已弃用，在PHP 7.0.0中已删除。
PERL样式正则表达式
Perl样式的正则表达式与POSIX非常相似。POSIX语法可与Perl样式的正则表达式函数互换使用。POSIX部分中介绍的量词也可以在PERL样式正则表达式中使用。
元字符
元字符是字母字符，后跟反斜杠，为组合赋予特殊含义。
例如，可以使用’d’元字符搜索大笔款项：/([d]+)000/。在这里/d将搜索数字字符的字符串。
以下是可在PERL样式正则表达式中使用的元字符列表-
| Character | Description |
|---|---|
| . | Matches a single character |
| s | It matches a whitespace character like space, newline, tab. |
| S | Non-whitespace character |
| d | It matches any digit from 0 to 9. |
| D | Matches a non-digit character. |
| w | Matches for a word character such as – a-z, A-Z, 0-9, _ |
| W | Matches a non-word character. |
| [aeiou] | It matches any single character in the given set. |
| [^aeiou] | It matches any single character except the given set. |
| (foo|baz|bar) | Matches any of the alternatives specified. |
修饰符
有多个修饰符可用，这使使用正则表达式的工作变得更加容易。例如-区分大小写或在多行中搜索等。
以下是PERL样式正则表达式中使用的修饰符列表-
| Character | Description |
|---|---|
| i | Makes case insensitive search |
| m | It specifies that if a string has a carriage return or newline characters, the $ and ^ operator will match against a newline boundary rather than a string boundary. |
| o | Evaluates the expression only once |
| s | It allows the use of .(dot) to match a newline character |
| x | This modifier allows us to use whitespace in expression for clarity. |
| g | It globally searches all matches. |
| cg | It allows the search to continue even after the global match fails. |
PHP Regexp POSIX函数
PHP当前提供了七个使用POSIX样式正则表达式搜索字符串的函数-
| Function | Description | |
|---|---|---|
| preg_match() | This function searches the pattern inside the string and returns true if the pattern exists otherwise returns false. | |
| preg_match_all() | This function matches all the occurrences of pattern in the string. | |
| preg_replace() | The preg_replace() function is similar to the ereg_replace() function, except that the regular expressions can be used in search and replace. | |
| preg_split() | This function exactly works like split() function except the condition is that it accepts regular expression as an input parameter for pattern. Mainly it divides the string by a regular expression. | |
| preg_grep() | The preg_grep() function finds all the elements of input_array and returns the array elements matched with regexp (relational expression) pattern. | |
| preg_quote() | Quote the regular expression characters. |
类别：PHP 技巧、
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