Detailed explanation of zero-width assertion examples of regular expressions

This article mainly introduces the zero-width assertion of regular expressions. It briefly introduces the concept, classification, PHP implementation skills and related precautions of zero-width assertion. Friends in need can refer to the example of this article.

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Preface

I have written an article about regular expressions before (http (://www.jb51.net/article/111359.htm) Regulars are introduced in detail in this article, but the introduction of zero-width assertions is rarely mentioned. Now add this content. In this article, we mainly address the following issues:

① What is a zero-width assertion and why should we use a zero-width assertion?
② How to use a zero-width assertion

Concept

Zero-width assertion, defined in most places, is used to find things before or after certain content (but not including these content), that is, they are like \b ^ $ \ Such anchoring is used to specify a location that should meet certain conditions (ie, assertions), so they are also called zero-width assertions. My understanding is that a matching search is performed before or after a string at a limited position. Therefore, the execution process of zero-width assertions is divided into two situations. If it is a forward assertion, it should be like this. The first step is to judge whether the assertion is true (that is, whether it meets certain conditions). The second step is to proceed to the next step if the conditions are met. Find matches in one step. If it is a reverse assertion, the first step is to match in the order of regular expressions. The second step is to determine whether reverse endorsement is met when encountering reverse endorsement.

Positive lookahead assertion

#What is a positive lookahead assertion is to search and match before the corresponding position in the string, using (?=exp) matching The position before exp.

Example

$str="abcgwcab";
$parent='/bc(?=gw)/';
$str=preg_match($parent,$str,$match);
var_dump($match);
/**
输出结果：
int 1
array (size=1) 
  0 => string 'bc' (length=2)
  */

Analysis: First find the gw position in the string "abcgwcab", assert it to be true, and then match bc. If the regular expression is written as $parent='/bc(?=gw)ca/'; the match will be unsuccessful.

Anti-lookahead assertion

What is an anti-lookahead assertion? Use (?!exp) to match something that is not followed by exp.

Example:

$str="abcgwcab";
$parent='/bc(?!ww)gw/';
$str=preg_match($parent,$str,$match);
var_dump($str);
var_dump($match);
/**
输出：
int 1
array (size=1)
 0 => string 'bcgw' (length=4)
*/

Analysis: First determine whether the string contains bc, then determine whether it is followed by ww, and finally match gw. It can be seen that other matching conditions can be added before and after the reverse assertion.

Assertion before and after

What is assertion before and after, it is to search and match after the corresponding position in the string, (?

Example:

$str="abcgwcab";
$parent=&#39;/(?<=gw)ca/&#39;;
$str=preg_match($parent,$str,$match);
var_dump($str);
var_dump($match);
/**
输出结果：
int 1
array (size=1)
 0 => string &#39;ca&#39; (length=2)
*/

Analysis: The first step is to find whether the character "abcgwcab" contains gw , the return result is true, and then proceed to the second step to find whether there is ca after gw.

Anti-posterior assertion

What is an anti-posterior assertion, use (?

$str="abcgwcab";
$parent=&#39;/(?<!bc)gw/&#39;;
$str=preg_match($parent,$str,$match);
var_dump($str);
var_dump($match);
/**
int 0
array (size=0)
 empty
*/

Analysis: First match gw in the string, and then determine whether it is preceded by bc. It is found that it is preceded by bc, so it returns false.

Exercise questions

Example 1:\d+(?=abc)
Determine whether a string contains 'abc' and whether it contains one or more numbers in front of it

Example 2: (? <br>Determine whether there are two consecutive numbers in the string, followed by a letter

Example 3 ：\d{3} (?!55)
Determine whether the string contains three consecutive numbers, and the three numbers are not followed by 55

Example 4: (? <br>Determine whether a string contains a combination of letters and numbers, and it is not preceded by ac

##Summary

What is the difference between preemptive assertions and late assertions?

Preemptive assertions are regular expressions before judgment assertions, and late assertions are before judgment assertions. Regular expressions, assertions come before other regular expressions. The first and last here are the order of other matching assertions in the regular expression.

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