Reg Regular expression learning notes

One 、 brief introduction

Regular expressions , Also known as regular expression .（ English ：Regular Expression, In code it is often abbreviated as regex、regexp or RE）, A concept of computer science . Regular expressions are often used for retrieval 、 Replace those that match a pattern ( The rules ) The text of .

Many programming languages support string manipulation with regular expressions . for example , stay Perl A powerful regular expression engine is built in . The concept of regular expression was originally developed by Unix Tool software in （ for example sed and grep） Popular . Regular expressions are usually abbreviated to “regex”, The singular number has regexp、regex, The plural is regexps、regexes、regexen.

Two 、 Usage rule

\d | matching 0-9 Any number in , Equivalent to [0-9]
\D | Match non numeric characters , Equivalent to [^0-9]
\w | Match any letter 、 Number or underscore , Equivalent to [^A-Za-z0-9_]
\W | With any non alphabetic 、 Match numbers or underscore characters , Equivalent to [^A-Za-z0-9_]
\s | Matches any whitespace characters , Including Spaces 、 tabs 、 Page identifier , Equivalent to ?[\f\n\r\t\v]
\S | Matches any non-whitespace characters , Equivalent to [^\f\n\r\t\v]
\n | Match newline
\r | Match a carriage return
\t | Match the match
\v | Match vertical tabs
\f | Match page breaks

These characters have special meanings in regular expressions , such as ：*,+,？,\,

\ | Escape character , Mark the next character as a special character
^ | Match the beginning of the string
$ | Match the end of the string

•  |  Zero or more matches the preceding character or subexpression 
  

• |  Matches the preceding character or subexpression one or more times 
  

? | Matches the preceding character or subexpression zero or once
. | “ spot ” Matching elimination “\r\n” Any single character other than
| | or
[ ] | Character set
( ) | grouping , To match parenthesis characters , Please use “(” ? or “)”

A finite character is also called a quantifier , Is used to indicate the number of characters to match .

‘’ | Zero or more matches the preceding character or subexpression
‘’ | Matches the preceding character or subexpression one or more times
? | Matches the preceding character or subexpression zero or once
{n} | n Is a non negative integer , Matched definite n Time
{n,} | n Is a nonnegative integer , Match at least n Time
{n,m} | n and m Is a nonnegative integer , among n<=m; Match at least n Time , at most m Time

Location characters are also called character boundaries , Tags match not characters but positions that meet certain conditions , So the positioning character is “ Zero width ”.

^ | Match the beginning of the string , To begin
$ | Match the end of the string , End of expression
\b | Matches a word boundary

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