Using word boundary instead of spaces in regex

Question

I'm trying to use regex to get rid of the first and last word in a sentence, but it only works when I'm using spaces to match, but it doesn't work when I'm trying to use word boundary. Why is that?

Using Spaces:

$inputX = "she hates my guts";
preg_match("~ .+ ~i", $inputX, $match);

print_r($match);

Result:

Array ( [0] => hates my )

Using Word Boundary:

$inputX = "she hates my guts";
preg_match("~\b.+\b~i", $inputX, $match);

print_r($match);

Result:

Array ( [0] => she hates my guts )

The . is greedy so .+? but then the result is she because start of string is a boundary. — AbraCadaver
– AbraCadaver, Commented Nov 5, 2015 at 22:16
@anubhava: Yes, but it's not preg_match_all() so only she. — AbraCadaver
– AbraCadaver, Commented Nov 5, 2015 at 22:17
why do you want to use word boundaries when you have a working regex with spaces? as @anubhava mentioned, \b will also match positions in the first and last word — Mariano
– Mariano, Commented Nov 5, 2015 at 22:27
By default, engines think of the beginning and end of string as a NON-word. — user557597
– user557597, Commented Nov 5, 2015 at 22:40

Lucas Trzesniewski · Accepted Answer · 2015-11-05 22:41:36Z

1

Here are the word boundaries:

 s h e   h a t e s   m y   g u t s 
^     ^ ^         ^ ^   ^ ^       ^

So your pattern matches like this:

 s h e   h a t e s   m y   g u t s 
^\_______________________________/^
|               |                 |
\b              .+                \b

If you want to get rid of the first and last word, I'd just replace these with an empty string, using the following pattern:

^\W*?\w+\s*|\s*\w+\W*$

Both \W* are there to account for possible punctuation (ie she hates my guts.) but you can remove them if they're not needed.

answered Nov 5, 2015 at 22:41

Lucas Trzesniewski

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1 Comment

user557597 Over a year ago

Hey, nice letter graphics!

Mariano · Accepted Answer · 2015-11-05 22:52:26Z

0

If you want to remove the first and last word in a sentence, you can:

explode() on spaces
remove the first and last element with array_slice()
implode() back again

Code

$inputX = "she hates my guts";
$result = implode(" ", array_slice(explode(" ", $inputX), 1, -1));

var_dump($result);

Output

string(8) "hates my"

answered Nov 5, 2015 at 22:52

Mariano

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