Regex-lookaround－Jemmy Walker

Regex當中的lookaround技巧是較少見到，會用卻是很有用，以下以Perl說明之：

my @nth = qw/1st 2nd 3rd 4th 5th 6th nth/; 被比較的內容1
my @nam = qw/jeffery jef jeff jeffs jefferson jefson/; 被比較的內容2

my @pattern_nth = qw/
(?=\dth)\d
(?!\dth)\d
(?<=\d)th
(?<!\d)th
/;

foreach $pattern (@pattern_nth) {
    print "$pattern : ";
    foreach (@nth) {
        print "$_ " if (/$pattern/);
    }
    print "\n";
}

my @pattern_nam = qw/
(?=jeffer)jef
(?!jeffer)jef
(?<=jef)son
(?<!jef)son
/;
print "\n";
foreach $pattern (@pattern_nam) {
    print "$pattern : ";
    foreach (@nam) {
        print "$_ " if (/$pattern/);
    }
    print "\n";
}

產出如下：

(?=\dth)\d : 4th 5th 6th 找出數字(\d)後是th的
(?!\dth)\d : 1st 2nd 3rd   找出數字(\d)後不是th的
(?<=\d)th : 4th 5th 6th     找出th前是數字(th)的 --> 與(?=\dth)結果雖同，但意義不同，括號是找尋符合右邊(或稱behind)的樣式
(?<!\d)th : nth                  找出th前不是數字(th)的

(?=jeffer)jef : jeffery jefferson     找出jef開頭而且叫jeffer
(?!jeffer)jef : jef jeff jeffs jefson 找出jef開頭但不叫jeffer的
(?<=jef)son : jefson          找出son前是叫jef的
(?<!jef)son : jefferson      找出son前不是前jef的 --> jeferrson

(?=<pattern>)與(?!<pattern>)為look ahead

(?<=<pattern>)與(?<!<pattern>)為look behind

曾經做過EDI的系統，EDI係以加號、冒號、單引號作為分隔字元，而問號則是跳脫字元，若要進行split，分隔字元就排除前面是問號的情況，以逗號為分隔字元的話，那pattern應該是(?<!\?),，因為問號本身也是Regex裡特殊字元，前面應當加斜線。