perl：使用替換添加到路徑

Question

我的腳本需要一個文件路徑，並且我想將一個目錄附加到路徑的末尾。問題是我想不確定論證是否有尾隨斜線。因此，例如：

$ perl myscript.pl /path/to/dir 
/path/to/dir/new 
$ perl myscript.pl /path/to/dir/ 
/path/to/dir/new 

我試過$path =~ s/\/?$/\/new/g，但導致雙/new如果斜線存在：

$ perl myscript.pl /path/to/dir 
/path/to/dir/new/new 
$ perl myscript.pl /path/to/dir 
/path/to/dir/new 

有什麼不對？

Answer 1

刪除該/g修改：

$path =~ s/\/?$/\/new/ 

工作正常。
你只想修改最後添加一個「新」，所以修改器沒有任何意義。

---

另外請注意，您可以使用不同的分隔符爲你的正則表達式：

$path =~ s{ /? $}{/new}x; 

是一點點清晰。

Answer 2

因爲/g是「全球性」，並會匹配其多次重複：

#!/usr/bin/env perl 
use strict; 
use warnings; 

#turn on debugging 
use re 'debug'; 

my $path = '/path/to/dir/'; 
$path =~ s/\/?$/\/new/g; 

print $path; 

第一替補，正則表達式引擎「左」標誌「行結束」後，並不需要匹配可選/。所以再次匹配。

例如爲：

Compiling REx "/?$" 
Final program: 
    1: CURLY {0,1} (5) 
    3: EXACT </> (0) 
    5: SEOL (6) 
    6: END (0) 
floating ""$ at 0..1 (checking floating) minlen 0 
Matching REx "/?$" against "/path/to/dir/" 
Intuit: trying to determine minimum start position... 
    doing 'check' fbm scan, [0..13] gave 13 
    Found floating substr ""$ at offset 13 (rx_origin now 12)... 
    (multiline anchor test skipped) 
    try at offset... 
Intuit: Successfully guessed: match at offset 12 
    12 <path/to/dir> </>  | 1:CURLY {0,1}(5) 
            EXACT </> can match 1 times out of 1... 
    13 <path/to/dir/> <>  | 5: SEOL(6) 
    13 <path/to/dir/> <>  | 6: END(0) 
Match successful! 
Matching REx "/?$" against "" 
Intuit: trying to determine minimum start position... 
    doing 'check' fbm scan, [13..13] gave 13 
    Found floating substr ""$ at offset 13 (rx_origin now 13)... 
    (multiline anchor test skipped) 
Intuit: Successfully guessed: match at offset 13 
    13 <path/to/dir/> <>  | 1:CURLY {0,1}(5) 
            EXACT </> can match 0 times out of 1... 
    13 <path/to/dir/> <>  | 5: SEOL(6) 
    13 <path/to/dir/> <>  | 6: END(0) 
Match successful! 
Matching REx "/?$" against "" 
Intuit: trying to determine minimum start position... 
    doing 'check' fbm scan, [13..13] gave 13 
    Found floating substr ""$ at offset 13 (rx_origin now 13)... 
    (multiline anchor test skipped) 
Intuit: Successfully guessed: match at offset 13 
    13 <path/to/dir/> <>  | 1:CURLY {0,1}(5) 
            EXACT </> can match 0 times out of 1... 
    13 <path/to/dir/> <>  | 5: SEOL(6) 
    13 <path/to/dir/> <>  | 6: END(0) 

這是因爲$是零寬度位置錨。如果沒有匹配，\/?也是如此。一旦模式一直消耗到尾部/並替換..然後正則表達式引擎繼續（因爲你告訴它與/g），並找到只剩$，因爲這仍然是行的末尾。這仍然是一個有效的替代。

但是，爲什麼不改用File::Spec：

#!/usr/bin/env perl 
use strict; 
use warnings; 
use File::Spec; 
use Data::Dumper; 

my $path = '/path/to/dir/'; 

my @dirs = File::Spec->splitdir($path); 

print Dumper \@dirs; 

$path = File::Spec->catdir(@dirs, "new"); 
print $path; 

這爲您提供了拆分和聯接路徑元素一個獨立於平臺的方式，並且不依賴於正則表達式匹配 - 這有各種方式就可以打破（如你找到的那個）。