逃脱成角度的支架的行为类似于前瞻 [英] Escaping Angled Bracket acts similar to look-ahead

问题描述

为什么转义转义尖括号>会表现出前瞻性的行为?

Why does escaping escaping the angled bracket > exhibit the look-ahead like behavior?

为清楚起见，我了解到尖括号不需要将其转义.
问题是，如何解释该模式以产生所显示的匹配项?

To be clear, I understand that the angled bracket does not necessitate being escaped.
The question is, how is the pattern being interpreted that it yields the match(es) shown

## match bracket, with or without underscore
## replace with "greater_"
strings <- c("ten>eight", "ten_>_eight")
repl    <- "greater_"

## Unescaped. Yields desired results
gsub(">_?", repl, strings)
#  [1] "tengreater_eight"  "ten_greater_eight"

## All four of these yield the same result
gsub("\\>_?",   repl, strings)  # (a)
gsub("\\>(_?)", repl, strings)  # (b)
gsub("\\>(_)?", repl, strings)  # (c)
gsub("\\>?",    repl, strings)  # (d)
gsub("\\>",     repl, strings)  # (e)
#  [1] "tengreater_>eightgreater_"   "ten_greater_>_eightgreater_"

gregexpr("\\>?", strings)

一些后续问题:

1.  Why do `(a)` and `(d)` yield the same result? 
2.  Why is the end-of-string matched?
3.  Why do none of `a, b, or c` match the underscore? 

推荐答案

\\>是单词边界(在左侧的单词字符和右侧的非单词字符或末尾匹配)线锚$的位置.

\\> is a word boundary Which matches between a word character(in the left side) and a non-word character (in the right side) or end of the line anchor $.

> strings <- c("ten>eight", "ten_>_eight")
> gsub("\\>", "greater_", strings)
[1] "tengreater_>eightgreater_"   "ten_greater_>_eightgreater_"

在上面的示例中，它仅匹配n之后的单词字符和非单词字符>之间的单词边界，然后匹配t与第一个元素中的行锚末端之间的边界.并且它在_(也是单词字符)和>之间匹配，然后在t和第二个元素中的行锚结尾(即$)之间匹配.最后，它将匹配的边界替换为您指定的字符串.

In the above example it match only the word boundary exists between a word character after n and a non-word character > then also the boundary between t and end of the line anchor in the first element. And it matches between _ (also a word character) and > then between t and end of the line anchor (ie, $) in the second element. Finally it replaces the matched boundaries with the string you specified.

一个简单的示例:

> gsub("\\>", "*", "f:r(:")
[1] "f*:r*(:"

请考虑以下输入字符串. ( w表示单词字符，N表示非单词字符)

Consider the below input string. (w means a word character, N means a non-word character)

    f:r(:
w___|||||
     |w|N
     N |
       |
       N

所以\\>之间匹配，

• f和:
• r和(

• f and :
• r and (

示例2:

> gsub("\\>", "*", "f") 
[1] "f*"

输入字符串:

f$
||----End of the line anchor
w

用*替换匹配的边界将得到以上结果.

Replacing the matched boundary with * will give the above result.

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