如何驗證正則表達式匹配的內容？

Question

在下面：

~$ perl -e ' 
my $var = "March 1998"; 
$var =~ /([0-9]*)/; 
print "$1\n"; 

沒有被打印。我不明白爲什麼，但想着這一點後，我認爲它，因爲它需要0或多次出現了一些和整條生產線滿足，只是不數部相匹配。但我怎麼能真正看到/「捕獲」它真正匹配，以驗證我的假設>

Answer 1

$1包含哪些被捕獲，所以你這樣做的權利。

特殊變量$',$&和$`也會有幫助。有關更多信息，請參閱perlvar。

$match = $var =~ /([0-9]*)/; 

print "There ", ($match ? "was" : "was not"), " a match\n"; 
print "The captured expression was '$1'\n"; 
print "The matched expression was '$&'\n"; # includes chars outside capture groups 
print "Expression before the match was '$`'\n"; 
print "Expression after the match was '$''\n"; 

用Perl 5.10起，您還可以使用/p修改和更具可讀性${^PREMATCH}，${^MATCH}，${^POSTMATCH}變量。

$match = $var =~ /([0-9]*)/p; 

print "There ", ($match ? "was" : "was not"), " a match\n"; 
print "The captured expression was '$1'\n"; 
print "The matched expression was '${^MATCH}'\n"; 
print "Expression before the match was '${^PREMATCH}'\n"; 
print "Expression after the match was '${^POSTMATCH}'\n"; 

Answer 2

正如已經提到的，從來沒有檢查$1，$2等不檢查匹配成功。否則，這些變量將包含最近成功的正則表達式匹配後所包含的任何變量。一個例外是當正則表達式可以匹配任何字符串 - 因爲你可以，但你可能沒有意識到這一點 - 但那種情況並不多見，這是更好地得到進入總測試賽的成功習慣。

@-和@+變量給出了匹配捕獲組的開始和結束索引（分別爲），該匹配捕獲組是上一次成功的正則表達式匹配的對象的字符串。

if ($var =~ /([0-9]*)/) { 
    print "The first capturing group matched from index $-[1] to index $+[1].\n"; 
} 

這將打印The first capturing group matched from index 0 to index 0.果然，你的字符串開頭的零位，所以這就是被匹配。