Detailed explanation of lookaround before and after regular expression tutorial_regular expression

This article mainly introduces regular expressionslookaround before and after the tutorial, and analyzes the implementation techniques and notes of the forward lookup and backward lookup functions based on specific problems. Friends in need can refer to

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

Note: In all examples, the regular expression matching results contain [ and ]## in the source text. #, some examples will be implemented using Java. If it is the usage of regular expressions in Java itself, it will be explained in the corresponding place. All java examples are tested under JDK1.6.0_13.

1. Introduction to the problem

In an HTML page, match the text between a pair of tags, such as matching the tags of the page, that is, < Text between title> and :

Text:

Regular expression:

<[Tt][Ii][Tt][Ll][Ee]>.*?

Result:

【】

Analysis: <[Tt][Ii][Tt][Ll][Ee]> means it is not case-sensitive. This pattern matches the title tag and the text between them, but it is 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 :, the pattern .+ matches any text, the 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 (?<=xxx)xxx. The backward search will be introduced next.

3. Backward search

Backward search

Operator is?<=. However, not all regular expression implementations support backward search, JavaScript does not support it, and the Java language supports backward search.

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:

(?<=\$)\d+(\.\d+)?

Result:

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

Analysis: (?< ;=\$) pattern matches $, \d+(\.\d+)? pattern matches integer or decimal. As can be seen from the results, the result does not include the currency symbol, but only the price. What would happen if we didn't use backward lookup? Use the pattern $\d+(\.\d+)?, which will include $ in the result. Using the pattern \d+(\.\d+)? will also match the numbers in category1(23), which is not what we want.

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. , *, + and other metacharacters.

4. Combine forward search and backward search

Use forward search and backward search together to solve the previous# Problem with text between ##HTML tags

: Text:

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

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

5. Find the difference between 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 whether The position of the text of the matched result that is returned (by specifying what text must be before and after the matched 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:

##(?<=)Positive backward search(?Negative backward search

(?=)	Correct Forward search
(?!)	Negative forward search

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:

(?<=\$)\d+

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 on this order.

Regular expression:

\b(?

Result:

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

Analysis: (?< ;!\$) represents a negative lookbehind search, which causes the result to contain only those values ​​that do not begin with $.

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 location where the text matching operation occurs, and achieve the effect of only matching without consumption.

PS: Here are two very convenient regular expression tools for your reference:

JavaScript regular expression online testing tool:
http://tools.jb51.net/regex/javascript

Regular expression online generation tool:
http://tools.jb51.net/regex/create_reg

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