Detailed explanation of lookaround before and after regular expression tutorial

The examples in this article describe the lookaround before and after the regular expression tutorial. Share it with everyone for your reference, the details are as follows:

Note: In all examples, the regular expression matching results are included between [and] in the source text. Some examples will be implemented using Java. If The usage of regular expressions in Java itself will be explained in the corresponding places. All java examples are tested under JDK1.6.0_13.

1. Introduction to the problem

In the HTML page, match the text between a pair of tags, such as matching the tags of the page, that is, between

Text between:

Text:

Regular expression: .*? [Tt][Ii][Tt][Ll][Ee]>

Result:

[welcome to my page]

Analysis: means case-insensitive, this The pattern matches the title tags and the text between them, but it's not perfect because we only want the text between the title tags, not the tags themselves. To solve this problem we need to use lookaround.

2. Forward search

Forward search specifies a pattern that must match but is not returned in the result. Forward search is actually a subexpression, which starts with ?=, and the text to be matched follows =.

Look at an example of matching the protocol part of a URL address:

Text: http://blog.csdn.net/mhmyqn

Regular expression: .+ (?=:)

Result: [http]://blog.csdn.net/mhmyqn

Analysis: The protocol part in the URL address is the part before:, pattern .+ matches Any text, subexpression (?=:) matches:, but the matched: does not appear in the result. We use ?= to indicate to the regular expression engine that as long as: is found, it will be fine, but it will not be included in the final return result. If you do not use forward matching (?=:) here, but use (:) directly, then the matching result will be http:, which includes:, which is not what we want.

Note: The front and back in front and back search refer to the relative position between the pattern and the text to be searched. The left is the front and the right is the back. That is, the forward search is: xxx(?=xxx), and the backward search is (?

3. Backward search

The backward search operator is ?

For example, if you want to find the price in the text (starting with $, followed by a number), the result does not contain the currency symbol:

Text: category1:$136.25,category2:$28,category3:$88.60

Regular expression: (?

Result: category1:$【136.25】,category2:$【28】,category3:$ 【88.60】

Analysis: (?

Note: The length of the forward search pattern is variable, and they can contain metacharacters such as ., *, +; while the backward search pattern can only be of fixed length and cannot contain ., *, Metacharacters like +.

4. Combine forward search and backward search

Use forward search and backward search together to solve the problem of text between the previous HTML tags:

Text:

Regular expression: (?).*?(?= [Tt][Ii][Tt][Ll][Ee]>)

Result:

Analysis: As can be seen from the results, the problem is perfectly solved. (?) is a backward operation, which matches

but does not consume it. The final matching result returned only contains the text between the tags.

5. Negate the forward and backward searches

The forward search and backward search mentioned above are usually used to match text, and their purpose is to determine the matching result that will be returned. The position of the text (by specifying what text must precede and follow the matching result). This usage is called forward search and forward search. There is also a negative forward search and a negative backward search, which find text that does not match a given pattern.

Operators for searching before and after:

For example, there is a price (starting with $, followed by a number) and quantity in a piece of text. We want to find the price and quantity. Let’s first look for the price:

Text: I paid $30 for 10 apples , 15 oranges, and 10 pears. I saved $5 on this order.

Regular expression: (?

Result: I paid 【$30】 for 10 apples , 15 oranges, and 10 pears. I saved 【$5】 on this order.

Find quantity:

Text: I paid $30 for 10 apples, 15 oranges, and 10 pears. I saved $5 onthis order.

Regular expression: \b(?

Result: I paid $30 for 【10】 apples, 【15】 oranges, and 【 10】pears. I saved $5 on this order.

Analysis: (?

6. Summary

With forward and backward search, you can have precise control over what content is included in the final matching result. The before and after search operation allows us to use subexpressions to specify the position where the text matching operation occurs, and achieve the effect of only matching without consumption.

I hope this article will be helpful for everyone to learn regular expressions.

For more regular expression tutorials, please pay attention to the PHP Chinese website for related articles before and after finding lookaround details!