First of all,here is a very useful regex tool : OSC在线正则表达式工具

1. What is Regular Expression?

A regular expression is a pattern which specifies a set of strings of characters; it is said to match certain strings.

--Ken Thompson

2. What can Regular Expression do?

1. Simple Pattern Matching

Regex ( abbreviation of regular expression,and it will be used in the follow artical ) are all about matching and finding patterns in text,from simple patterns to the very complex,for example:

• matching string literals
• matching digits : [0-9]
• matching non-digits : [^\d],which is the same as [^0-9]
• matching word and non-word characters : \w means matching all word characters and it's the same as [a-zA-Z0-9].Use \W or [^a-zA-Z0-9] to match a non-word character.
• matching whitespace : use \s to match Spaces,Tab(\t),Line Feeds(\n) and Carriage returns(\r). \S can help to match a non-whitespace character,which means [^ \t\n\r] or [^\s].
• another way to match all characters :".",and the number of this dot means the length of the charaters to match. You can also use .{8} ( in the brace you can put any numbers ). Of course,here we can also set some word boundaries like this: \bR.{3}x\b can match Regex if there is a Regex in your text.
• marking up the text : there will be more later in this article.

2. Boundaries

In this part,I am going to talk about zero-width assertions. It does not match a character,but rather a location in a string. Some of these zero-width assertions such as ^ and $,are also called anchors. Here are some boundaries i am talking about:

• the beginning and end of a line:
• word boundaries:
• the beginning and end of a subject:
• boundaries that quote string liberals: