Regex Character Class Subtraction with PHP

Question

HI,

I'm trying to match UK postcodes, using the pattern from http://interim.cabinetoffice.gov.uk/media/291370/bs7666-v2-0-xsd-PostCodeType.htm ,

/^[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][A-Z-[CIKMOV]]{2}$/

I'm using this in PHP, but it doesn't match the valid postcode OL13 0EF . This postcode does match, however, when I remove the -[CIKMOV] character class subtraction.

I get the impression that I'm doing character class subtraction wrong in PHP. I'd be most grateful if anyone could correct my error.

Thanks in advance for your help.

Ross

Answer 1

Most of the regex flavours do not support character class subtraction. Instead you could use look-ahead assertion:

/^[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9](?!.?[CIKMOV])[A-Z]{2}$/

Answer 2

If class subtraction is not supported, you should be able to use negative classes to achieve subtractions.

Some examples are [^\\D] = \\d , [^[:^alpha:]] = [a-zA-Z]

Your problem could be solved like that, using a negative POSIX character class inside a character class like [^az[:^alpha:]CIKMOV]

[^
az # not az
[:^alpha:] # not not A-Za-z
CIKMOV # not C,I,K,M,O,V
]

Edit - This works too and might be easier to read: [^[:^alpha:][:lower:]CIKMOV]

[^
[:^alpha:] # A-Za-z
[:lower:] # not az
CIKMOV # not C,I,K,M,O,V
]

The result is a character class that is AZ without C,I,K,M,O,V
basically a subtraction.

Here is a test of 2 different class concoctions (in Perl):

use strict;
use warnings;

my $match = '';

   # ANYOF[^\0-@CIKMOV[-\377!utf8::IsAlpha]
for (0 .. 255) {
   if (chr($_) =~ /^[^a-z[:^alpha:]CIKMOV]$/) {
       $match .= chr($_); next;
   }
   $match .= ' ';
}
$match =~ s/^ +//;
$match =~ s/ +$//;
print "'$match'\n";
$match = '';

   # ANYOF[^\0-@CIKMOV[-\377+utf8::IsDigit !utf8::IsWord]
for (0 .. 255) {
   if (chr($_) =~ /^[^a-z\d\W_CIKMOV]$/) {
       $match .= chr($_); next;
   }
   $match .= ' ';
}
$match =~ s/^ +//;
$match =~ s/ +$//;
print "'$match'\n";

Output shows the discontinuation in AZ minus CIKMOV, from tested ascii characters 0-255:
'AB DEFGH JLN PQRSTU WXYZ'
'AB DEFGH JLN PQRSTU WXYZ'

Answer 3

PCRE does not support char class subtraction.

So you can enumerate all the uppercase letters except CIKMOV :

^[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][ABDEFGHJLNPQRSTUWXYZ]{2}$

which can be shorted using range as:

^[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][ABD-JLNP-UW-Z]{2}$

Answer 4

I think you're going to have to replace [AZ-[CIKMOV]] with [ABD-HJLNP-UW-Z] . I don't think php supports character class substraction. My alternative reads something like "A, B, D to H, J, L, N, P to U, and W to Z".