A question about groups in regular expressions

[moroshko]

Can someone tell me please why in the following program $1 is not defined ? I thought that every group (every pair of parentheses) is going to $1, $2, $3 and so on.
Where am I wrong ?
Which pairs of parentheses are going to $1, $2, $3, ... ?
Thanks in advance !!


$_ = "a bc";

if (/(bc)?/) {
print "\$1 = [$1]\n" if (defined $1);
} else {
print "No match !";
};

 

[KevinADC]

The problem in your example appears to be the quantifier "?". I am not sure why that is a problem though. If you do this:

$_ = "a bc";

if (/(bc)/) {
   print "\$1 = [$1]\n" if (defined $1);
} else {
   print "No match !";
};

$1 gets defined. If you do this:

$_ = "a bc";

if (/(bc)?/) {
   print "Matched 1\n";
   print "\$1 = [$1]\n" if (defined $1);
} else {
   print "No match !";
};

"Matched !" is printed but $1 is not defined. The quantifier is interfering with the pattern memory but not the matching. Maybe someone else can explain why.

------------------------------------------
- Kevin, perl coder unexceptional!

 

[prex1]

[tt]?[/tt] means '[tt]Match 1 or 0 times[/tt]', so the regex matches always on nothing at the beginning of the string (use [tt]pos()[/tt] to check).

Franco

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