/c对于tr和m的作用是不同的：
$pathname =˜ tr/a-zA-Z/_/cs; # change non-(ASCII)alphas to single underbar
"abc" =~ /\Aabc\z/gc;# c avoids position reset
tr等价于
y/SEARCHLIST/REPLACEMENTLIST/cds
这里a-zA-Z并不是regex,只是SEARCHLIST。
同样这里的/c表示Complement the SEARCHLIST
再说下/gc修饰符对于regex的作用
perlre描述如下：
g and c Global matching, and keep the
Current position after failed
matching. Unlike i, m, s and x, these
two flags affect the way the regex is
used rather than the regex itself. See
Using regular expressions in Perl in
perlretut for further explanation of
the g and c modifiers.
即对于m，/c使得在匹配失败时pos不被重置为undef
看个例子：
#!/usr/bin/perl
use strict;
use warnings;
my $s = "abc";
pos $s = 0;
study $s;
$s =~ /a/gc;
$s =~ /"d"/gc;
print pos $s;
输出：
1
而
#!/usr/bin/perl
use strict;
use warnings;
my $s = "abc";
pos $s = 0;
study $s;
$s =~ /a/gc;
$s =~ /"d"/g;
print pos $s;
输出：
Use of uninitialized value in print at oh line 11.
背景知识：
关于前面说的pos
指的是匹配首末字符的offset信息,
保存在@-/@+这两个数组中
#!/usr/bin/perl
use strict;
use warnings;
my $s = "abc";
pos $s = 0;
study $s;
$s =~ /a/gc;
print "The entire match began at $-[0] and ended at $+[0]\n";
=anything
print "The first match began at $-[1] and ended at $+[1]\n";
print "The second match began at $-[2] and ended at $+[2]\n";
=cut
输出：
The entire match began at 0 and ended at 1
0
