On 03/13/2017 02:06 PM, ToddAndMargo wrote:
Hi All,
$ perl6 -e 'my $x="ab12cd"; $x ~~ m/ab(1q2)cd/; say "$x\n\$0=<$0>\n";'
Use of Nil in string context in block <unit> at -e line 1
ab12cd
$0=<>
With out the "q" in this, it works. I deliberately put
the "q" to see what would happen when a patter was not
found.
Is there a way around the "use of nil" finger wag
if a patter is not found?
Or should I always test for its presence first if
there is a possibility the pattern might not exist?
Many thanks,
-T
Follow up. My notes on matching:
Thank you all for the help!
Perl 6: Pattern Matching:
This Perl 5:
$ perl -e 'my $x="abc123def"; $x =~ m[(abc)(123)(def)]; print
"$x\n\$1=<$1> \$2=<$2> \$3=<$3>\n";'
abc123def
$1=<abc> $2=<123> $3=<def>
Translates in Perl 6 to:
Note: Perl 5 starts counting at 1 and Perl 6 starts counting at 0
$ perl6 -e 'my $x="abc123def"; $x ~~ m/(abc)(123)(def)/; say
"$x\n\$0=<$0> \$1=<$1> \$2=<$2>\n";'
abc123def
$0=<abc> $1=<123> $2=<def>
perl6 -e 'my $x="abc\(123\)def"; $x ~~ m/(abc)\((123)\)(def)/; say
"$x\n\$0=<$0> \$1=<$1> \$2=<$2>\n";'
abc(123)def
$0=<abc> $1=<123> $2=<def>
ooops! No match!
$ perl6 -e 'my $x="ab12cd"; $x ~~ m/ab(1q2)cd/; say "$x\n\$0=<$0>\n";'
Use of Nil in string context
in block <unit> at -e line 1
ab12cd
$0=<>
This is because you are trying to print out undefined variables.
To work around this:
Reverse:
perl6 -e 'my $x="ab12cd"; say "$x\n\$0=<$0>\n" if $x ~~
m/ab(1q2)cd/;'
Forward:
perl6 -e 'my $x="ab12cd"; if $x ~~ m/ab(1q2)cd/ { say
"$x\n\$0=<$0>\n"};'
Note: "ALL" matches have to be satisfied, or it will return "false"
pre-assigning variables, well sort of. You can not assign an external
variable, like you can with a "for" loop.
Note: the name of the variable is $<a> not $a
perl6 -e 'my $x="ab12cd"; $x ~~ m/$<a>=(ab) $<b>=(12) $<c>=(cd)/; say
"$x\n\$a=<$<a>>\t\$b=<$<b>>\t\$c=<$<c>>\n";'
ab12cd
$a=<ab> $b=<12> $c=<cd>
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