Another way: 

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (qw(15556667777  3334445555  999999999  88888888  121212121
+212)) {
   printf qq{'$s' -> };
   my ($npa, $nxx) = $s =~ m{ \A 1? (\d{3}) (\d{3}) \d{4} \Z }xms;
   print defined($nxx) ? qq{npa '$npa', nxx '$nxx'} : 'invalid number'
+;
   }
"
'15556667777' -> npa '555', nxx '666'
'3334445555' -> npa '333', nxx '444'
'999999999' -> invalid number
'88888888' -> invalid number
'121212121212' -> invalid number

[download]
Please see perlre, perlretut, and perlrequick.

Update: This approach to parsing is open to further regexish elaboration (I took a quick look at WP for NPA and NXX definitions, but I'm sure it's more involved): 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $rx_npa  = qr{ [2-9] (?! 11) \d\d }xms;
 my $rx_nxx  = qr{ [2-9] (?! 11) \d\d }xms;
 my $rx_subs = qr{ \d{4} }xms;
 ;;
 for my $s (qw(
   15556667777 x15156667777 15516667777x x15556167777x
   3334415555  x3134145555  3314415555x  x3334445555x
   999999999 x999999999 999999999x x999999999x
   88888888  x88888888  88888888x  x88888888
   121212121212 x121212121212 121212121212x x121212121212x
   5119999999 15119999999  9995119999 19995119999
   )) {
   printf qq{'$s' -> };
   my $got_pn =  my ($npa, $nxx, $subs) = $s =~ m{
     (?<! \d) 1? ($rx_npa) ($rx_nxx) ($rx_subs) (?! \d)
     }xms;
   print $got_pn ? qq{npa '$npa', nxx '$nxx', subscriber '$subs'} : 'n
+o number';
   }
"
'15556667777' -> npa '555', nxx '666', subscriber '7777'
'x15156667777' -> npa '515', nxx '666', subscriber '7777'
'15516667777x' -> npa '551', nxx '666', subscriber '7777'
'x15556167777x' -> npa '555', nxx '616', subscriber '7777'
'3334415555' -> npa '333', nxx '441', subscriber '5555'
'x3134145555' -> npa '313', nxx '414', subscriber '5555'
'3314415555x' -> npa '331', nxx '441', subscriber '5555'
'x3334445555x' -> npa '333', nxx '444', subscriber '5555'
'999999999' -> no number
'x999999999' -> no number
'999999999x' -> no number
'x999999999x' -> no number
'88888888' -> no number
'x88888888' -> no number
'88888888x' -> no number
'x88888888' -> no number
'121212121212' -> no number
'x121212121212' -> no number
'121212121212x' -> no number
'x121212121212x' -> no number
'5119999999' -> no number
'15119999999' -> no number
'9995119999' -> no number
'19995119999' -> no number

[download]

Give a man a fish:  <%-{-{-{-<

In reply to Re: Negative Index with "substr" Not Working by AnomalousMonk
in thread Negative Index with "substr" Not Working by he204035

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