The zero width assertion of JS regular

The regular expression matches that we've learned are all "width" and will match with \w+。 The following text: 。

Regular Expression  

If you do not want to match the symbol, match only one position, use the "0 wide assertion" (Match width is zero, satisfy certain condition/assertion), 0 wide assertion use (? = expression) syntax, \w+(?=。) For example, where (?=。) represents 。 the previous position (antecedent assertion)

Regular expression.  

If you need to match a later location, such as:

。 regular . expression  

Then use (?<=。) (?<=。)\w+ The post assertion to get the above matching result

Use (?<=<b>).*(?=</b>) content from a matching tag

<b> Bold </b>  

Negative 0 Wide Assertion

Negative to 0 wide assertion (?! Expression) is also a position that matches a 0 width, but the "assertion" of the position takes the inverse of the expression (?!表达式) , 表达式 For example to indicate the 表达式 previous position, if not, Match this position, if 表达式 It is, it does not match:

expression expression,expression, expression.       

The above is .+n(?!。) the matching result used. Note the difference from the match .+n[^。]

Expressionexpression,expression, expression.    

Similarly, negative 0-wide assertions also have "antecedent" and "post-hair" two, negative 0-wide after the assertion is (? <! expression)

Use (?<![</])para(?!>) match text below

<para>para represents a paragraph </para>    

• (?<![</])Indicates that para the < left cannot be or / (?!>) is not the para right>

The zero width assertion of JS regular