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Backreference for regular expression Learning
This article describes how to use a regular expression to reference backreference. We will share this with you for your reference. The details are as follows:
In all examples, the regular expression matching result is included between [and] in the source text. In some examples, Java is used. If the regular expression is used in java, it will be described in the corresponding area. All java examples have passed the test under JDK1.6.0 _ 13.
I. Problem Introduction
A question about matching title tags (H1-H6) on an HTML page:
Text:
<body>Regular Expression: <[hH] [1-6]> .*? </[HH] [1-6]>
Result:
<Body>
[<H1> Welcome to my page </H1>]
Content is divided into twosections: <br>
[<H2> Introduction Information about me.
[<H2> holobby </H2>]
Information about my holobby.
[<H2> This is invalid HTML </Body>Analysis: the mode <[hH] [1-6]> matches the start tag of any level-1 title and is case insensitive, in this example, it matches
Ii. backtracking and reference matching
Backend Reference refers to the subexpression defined in the first half of a pattern. The usage, division, and reference of subexpressions have been described earlier. Here we will solve the preceding example:
Text:
<body>Regular Expression: <[hH] ([1-6])> .*? </[HH] \ 1>
Result:
<Body>
[<H1> Welcome to my page </H1>]
Content is divided into twosections: <br>
[<H2> Introduction Information about me.
[<H2> holobby </H2>]
Information about my holobby.
<H2> This is invalid HTMLAnalysis: first, match the start title label mode <[hH] ([1-6])>, and use [1-6] As a subexpression using parentheses, the tag mode of the matched ending title is </[hH] \ 1>, where \ 1 indicates referencing the first subexpression ([1-6]), if ([1-6]) matches 1, \ 1 also matches 1. If it matches 2, \ 1 matches 2, therefore, the last invalid title tag will not be matched.
PS: here we will provide two very convenient Regular Expression tools for your reference:
JavaScript Regular Expression online testing tool:
Http://tools.jb51.net/regex/javascriptRegular Expression generation tool:
Http://tools.jb51.net/regex/create_reg
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