C# 正则表达式 - 只有在子字符串存在时才匹配? [英] C# Regex - only match if substring exists?

问题描述

好的，所以我想我已经掌握了否定的方法 - 现在只选择一个包含指定子字符串的匹配怎么办?

Ok, so I think I've got a handle on negation - now what about only selecting a match that has a specified substring within it?

给定:

This is a random bit of information from 0 to 1.
  This is a non-random bit of information I do NOT want to match
This is the end of this bit

This is a random bit of information from 0 to 1.
  This is a random bit of information I do want to match
This is the end of this bit

并尝试以下正则表达式:

And attempting the following regex:

/(?s)This is a random bit(?:(?=This is a random).)*?This is the end/g

为什么这不起作用?我错过了什么?

Why isn't this working? What am I missing?

我正在使用 regexstorm.com 进行测试...

I'm using regexstorm.com for testing...

推荐答案

你把消极的前瞻变成了积极的前瞻，这毁了一个温和的贪婪令牌.它不会这样工作，因为正向前瞻要求文本在 This is a random bit 之后的每个位置都等于 This is a random.

You ruined a tempered greedy token by turning the negative lookahead into a positive one. It won't work that way because the positive lookahead requires the text to equal This is a random at each position after This is a random bit.

您需要:

• 匹配前导分隔符(这是一个随机位)
• 匹配所有 0+ 不是前导/结束定界符的文本，也不是该块内所需的随机文本
• 匹配里面的特定字符串(This is a random)
• 匹配所有 0+ 不是前导/结束定界符的文本
• 匹配结束分隔符(这是结束)

所以，使用

(?s)This is a random bit(?:(?!This is a random bit|This is the end|This is a random).)*This is a random(?:(?!This is a random bit|This is the end).)*This is the end

见正则表达式演示

• (?s) - (. 上的 DOTALL 模式匹配换行符)
• 这是一个随机位 - 前导分隔符
• (?: # 调节贪婪令牌的开始(?!这是一个随机位#前导分隔符|这是结尾#尾随定界符|这是一个随机的)#里面的特定字符串.# 任意字符)* # 缓和贪婪令牌结束
• 这是一个随机的 - 指定的子串
• (?:(?!This is a random bit|This is the end).)* - 另一个缓和的贪婪令牌匹配任何不前导/结束分隔符直到第一个的文本......
• 到此结束 - 尾随分隔符

• (?s) - DOTALL mode on (. matches a newline)
• This is a random bit - Leading delimiter
• (?: # Start of the tempered greedy token (?!This is a random bit # Leading delimiter | This is the end # Trailing delimiter | This is a random) # Sepcific string inside . # Any character )* # End of tempered greedy token
• This is a random - specified substring
• (?:(?!This is a random bit|This is the end).)* - Another tempered greedy token matching any text not leading/closing delimiters up to the first...
• This is the end - trailing delimiter

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