Regex match entire words only

I have a regex expression that I'm using to find all the words in a given block of content, case insensitive, that are contained in a glossary stored in a database. Here's my pattern:

/($word)/i

The problem is, if I use /(Foo)/i then words like Food get matched. There needs to be whitespace or a word boundary on both sides of the word.

How can I modify my expression to match only the word Foo when it is a word at the beginning, middle, or end of a sentence?

---

Use word boundaries:

/b($word)b/i

Or if you're searching for "SPECTRE" like in Sinan Ünür's example:

/(?:W|^)(Q$wordE)(?:W|$)/i

---

To match any whole word you would use the pattern (w+)

Assuming you are using PCRE or something similar:

Above screenshot taken from this live example: http://regex101.com/r/cU5lC2

Matching any whole word on the commandline with (w+)

I'll be using the phpsh interactive shell on Ubuntu 12.10 to demonstrate the PCRE regex engine through the method known as preg_match

Start phpsh, put some content into a variable, match on word.

el@apollo:~/foo$ phpsh

php> $content1 = 'badger'
php> $content2 = '1234'
php> $content3 = '$%^&'

php> echo preg_match('(w+)', $content1);
1

php> echo preg_match('(w+)', $content2);
1

php> echo preg_match('(w+)', $content3);
0

The preg_match method used the PCRE engine within the PHP language to analyze variables: $content1 , $content2 and $content3 with the (w)+ pattern.

$content1 and $content2 contain at least one word, $content3 does not.

Match a number of literal words on the commandline with (dart|fart)

el@apollo:~/foo$ phpsh

php> $gun1 = 'dart gun';
php> $gun2 = 'fart gun';
php> $gun3 = 'farty gun';
php> $gun4 = 'unicorn gun';

php> echo preg_match('(dart|fart)', $gun1);
1

php> echo preg_match('(dart|fart)', $gun2);
1

php> echo preg_match('(dart|fart)', $gun3);
1

php> echo preg_match('(dart|fart)', $gun4);
0

variables gun1 and gun2 contain the string dart or fart. gun4 does not. However it may be a problem that looking for word fart matches farty . To fix this, enforce word boundaries in regex.

Match literal words on the commandline with word boundaries.

el@apollo:~/foo$ phpsh

php> $gun1 = 'dart gun';
php> $gun2 = 'fart gun';
php> $gun3 = 'farty gun';
php> $gun4 = 'unicorn gun';

php> echo preg_match('(bdartb|bfartb)', $gun1);
1

php> echo preg_match('(bdartb|bfartb)', $gun2);
1

php> echo preg_match('(bdartb|bfartb)', $gun3);
0

php> echo preg_match('(bdartb|bfartb)', $gun4);
0

So it's the same as the previous example except that the word fart with a b word boundary does not exist in the content: farty .

---

Using b can yield surprising results. You would be better off figuring out what separates a word from its definition and incorporating that information into your pattern.

#!/usr/bin/perl

use strict; use warnings;

use re 'debug';

my $str = 'S.P.E.C.T.R.E. (Special Executive for Counter-intelligence,
Terrorism, Revenge and Extortion) is a fictional global terrorist
organisation';

my $word = 'S.P.E.C.T.R.E.';

if ( $str =~ /b(Q$wordE)b/ ) {
    print $1, "n";
}

Output:

Compiling REx "b(S.P.E.C.T.R.E.)b"
Final program:
   1: BOUND (2)
   2: OPEN1 (4)
   4:   EXACT  (9)
   9: CLOSE1 (11)
  11: BOUND (12)
  12: END (0)
anchored "S.P.E.C.T.R.E." at 0 (checking anchored) stclass BOUND minlen 14
Guessing start of match in sv for REx "b(S.P.E.C.T.R.E.)b" against "S.P
.E.C.T.R.E. (Special Executive for Counter-intelligence,"...
Found anchored substr "S.P.E.C.T.R.E." at offset 0...
start_shift: 0 check_at: 0 s: 0 endpos: 1
Does not contradict STCLASS...
Guessed: match at offset 0
Matching REx "b(S.P.E.C.T.R.E.)b" against "S.P.E.C.T.R.E. (Special Exec
utive for Counter-intelligence,"...
   0           |  1:BOUND(2)
   0           |  2:OPEN1(4)
   0           |  4:EXACT (9)
  14      |  9:CLOSE1(11)
  14      | 11:BOUND(12)
                                  failed...
Match failed
Freeing REx: "b(S.P.E.C.T.R.E.)b"

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