Regular expression syntax tutorial (including online testing tool)-headlines-php.cn

Regular expression syntax tutorial (including online testing tool)

What is a regular expression?

A regular expression is a group of letters and symbols special text, which can be used to find sentences that meet the format you want from the text.

A regular expression is when matching strings from left to right in a body string a style of. The term "Regular expression" is a bit tricky to pronounce, and we often use the abbreviated terms "regex" or "regexp". Regular expressions can replace strings in text according to certain matching patterns from a basic string, validate forms, extract strings, etc.

Imagine you are writing an application, and then you want to set an User naming rules allow user names to contain characters, numbers, underscores and hyphens, and limit the number of characters so that the name does not look so ugly. We use the following regular expression to verify a username:

Regular expression syntax tutorial (including online testing tool)

The above regular expression can accept john_doe, jo-hn_doe, john12_as. But it does not match Jo, because it contains uppercase letters and is too short.

Directory

　1. Basic matching

　2. Metacharacters

　　2.1 Dot operator.

　　 2.2 Character set

　　　2.2.1 Negative character set

　　2.3 Number of repetitions

　　　2.3. #1 }

　　2.5 (...) Characteristic group

　　2.6 | Or operator

　　2.7 Convert special characters

　　2.8 Anchor point

#2.8.2 $

#　3. Abbreviated character set

　4. Zero-width assertion (before and after preview)

　　　4.1 ?=...Positive lookahead assertion

　　4.2 ?!... Negative lookahead assertion

　　4.3 ?

　　4.4 ?

　5. Flag

　　5.1 Case Insensitive

　　5.2 Global search(Global search)

　　5.3 Multiline modifier (Multiline)

##1. Basic matching

Regular expression actually This is the format used when performing a search, and is a combination of letters and numbers. For example: a regular expression the, which represents a rule: starting with the letters

, followed by

Online testing tool

Regular expression123matches the string123. It matches the entered regular expression character by character Do comparison.

Regular expressions are case-sensitive, so The will not match the.

"The" => The fat cat sat on the mat.

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Online testing tool

2. Metacharacters

Regular expressions mainly rely on metacharacters. Metacharacters do not represent their literal meaning, they all have special meanings. Some metacharacters have special meanings when written in square brackets. The following is an introduction to some metacharacters:

## Matches >= 0 repeated characters before the * sign.Match >= 1 repeated character before the number.?Mark? The preceding characters are optional. {n,m} Matches num characters before the braces (n <= num <= m). (xyz)Character set, matching strings that are exactly equal to xyz.| or operator, matches the characters before or after the symbol. \ escape character, used to match some reserved characters ^ Match from the beginning line. $Match from the end.

2.1 点运算符 `.`

　　.是元字符中最简单的例子..匹配任意单个字符, 但不匹配换行符. 例如, 表达式.ar匹配一个任意字符后面跟着是a和r的字符串.

".ar" => The car parked in the garage.

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Metacharacters	Description
.	Period matches any single character except newline.
[ ]	Character type. Matches any character within square brackets.
[^ ]	Negative character type. Matches any characters except square brackets
*





	[ ] ( ) { } . * ? ^ $ \ \|

简写	描述
.	除换行符外的所有字符
\w	匹配所有字母数字, 等同于 `[a-zA-Z0-9_]`
\W	匹配所有非字母数字, 即符号, 等同于: `[^\w]`
\d	匹配数字: `[0-9]`
\D	匹配非数字: `[^\d]`
\s	匹配所有空格字符, 等同于: `[\t\n\f\r\p{Z}]`
\S	匹配所有非空格字符: `[^\s]`
\f	匹配一个换页符
\n	匹配一个换行符
\r	匹配一个回车符
\t	匹配一个制表符
\v	匹配一个垂直制表符
\p	匹配 CR/LF (等同于 `\r\n`)，用来匹配 DOS 行终止符

符号	描述
?=	正先行断言-存在
?!	负先行断言-排除
?<=	正后发断言-存在
?	负后发断言-排除

Flags	Description
i	Ignore case.
g	Global search.
m	Multiple lines: Anchor metacharacter`^` `$` The working range is at the beginning of each line.

Regular expression syntax tutorial (including online testing tool)

2. Metacharacters

2.1 点运算符 .

2.2 字符集

2.2.1 否定字符集

2.3 重复次数

2.3.1 * 号

2.3.2 + 号

2.3.3 ? 号

2.4 {} 号

2.5 (...) 特征标群

2.6 | 或运算符

2.7 转码特殊字符

2.8 锚点

2.8.1 ^ 号

2.8.2 $ 号

3. 简写字符集

4. 零宽度断言(前后预查)

4.1 ?=... 正先行断言

4.2 ?!... 负先行断言

4.3 ?<= ... 正后发断言

4.4 ?<!... 负后发断言

5. Flags

5.1 忽略大小写 (Case Insensitive)

5.2 全局搜索 (Global search)

5.3 多行修饰符 (Multiline)

6. 贪婪匹配与惰性匹配 (Greedy vs lazy matching)

2.1 点运算符 `.`

2.3.1 `*` 号

2.3.2 `+` 号

2.3.3 `?` 号

2.4 `{}` 号

2.5 `(...)` 特征标群

2.6 `|` 或运算符

2.8.1 `^` 号

2.8.2 `$` 号

4.1 `?=...` 正先行断言

4.2 `?!...` 负先行断言

4.3 `?<= ...` 正后发断言

4.4 `?<!...` 负后发断言