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Perl | Regex 中的断言

原文:https://www.geeksforgeeks.org/perl-assertions-in-regex/

正则表达式(Regex 或 RE) 在 Perl 中是指描述给定字符串中的序列或搜索模式的特殊字符串。 正则表达式中的断言是指匹配在某种程度上是可能的。Perl 的 regex 引擎从左到右评估给定的字符串，搜索序列的匹配，当我们在字符串中的给定点找到匹配序列时，该位置被称为匹配位置或当前匹配位置。环视断言便于我们在不改变当前匹配位置的情况下，在匹配位置或当前匹配位置之前或之后匹配模式或字符串。

断言类型: Regex 中主要有两种断言类型，进一步划分: Types of Assertions

  1. Lookahead Assertions: In this type of assertion we look ahead the current match position, means will search the string after the current match position or the pattern/string succeeding string, without moving the current match position.

前瞻断言的语法:

/(?=pattern)/

前瞻断言可以进一步分为:

  • 正向前瞻(？=pattern): 正向前瞻就像正常的前瞻断言一样，它确保模式确实存在于我们正在匹配的给定字符串中。 例:
# Perl code for demonstrating 
# Positive Lookahead Modules used 
use strict; 
use warnings; 

$_ = "chicken cake";

# It will check that if the cake is
# in the string, then 
# replace the chicken with chocolate
s/chicken (?=cake)/chocolate /;

# printing the result
print $_;

输出:

chocolate cake
  1. Negative Lookahead(?!pattern): In Negative Lookahead, it Lookaheads the pattern and makes sure that the pattern doesnot exist in the given string which we are matching. To make Lookahead assertion as Negative Lookahead we just replace the ‘='(Equal) with ‘!'(Exclamation).

Example:

# Perl code for demonstrating 
# Negative Lookahead Modules used 
use strict; 
use warnings; 

$_ = "chicken cake";

# it will check that if the curry is not
# in the given string, then
# replace the chicken with chocolate
s/chicken (?!curry)/chocolate /;

# printing the result
print $_;

输出:

chocolate cake
  1. Lookbehind Assertions: In this type of assertion we look behind the current match position, means will search the string before the current match position or the preceding string, without moving the current match position. Syntax for Lookbehind Assertions:
/(?<=pattern)/

后视断言可以进一步分为:

  • Positive Lookbehind(?<=pattern):Positive Lookbehind, it makes sure that the pattern does exist in the given string we are matching. Example:
# Perl code for demonstrating 
# Positive Lookbehind Modules used 
use strict; 
use warnings; 

$_= "chocolate curry";

# it will check that if the chocolate 
# is preceding curry, then 
# it will replace the curry with cake
s/(?<=chocolate )curry/cake /;

# printing the result 
print $_;

输出:

chocolate cake
  • Negative Lookbehind(?<!pattern):Negative Lookbehind, it Lookbehind the pattern and make sure that the pattern does not exist in the given string we are matching. To make Lookbehind assertion as negative we just replace the ‘='(Equal) with ‘!'(Exclamation).

示例:

# Perl code for demonstrating 
# Negative Lookbehind Modules used 
use strict; 
use warnings; 

$_= "chocolate curry";

# it will check that if the chicken
# is not preceding curry, then 
# it will replace the curry with cake
s/(?<!chicken )curry/cake /;

# printing the result 
print $_;

输出:

chocolate cake
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