# Regular Expression: How the rule is applied for the Pattern '/((red|white) (king|queen))/...

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• Author
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Can any one explain how the rule is applied for the following Regular
expression

\$Str = 'the red king';

\$Pattern = '/((red|white) (king|queen))/';

preg_match(\$Pattern,\$Str,\$Val);

Result:

Array
(
[0] => red king
[1] => red king
[2] => red
[3] => king
)

Moses

## Re: Regular Expression: How the rule is applied for the Pattern '/((red|white) (king|queen))/'

On Tue, 22 Jan 2008 12:41:26 +0100, mosesdinakaran@gmail.com  =

\$Pattern = '/(    # start of capture no.1
(        # start of capture no.2
red|white    # either literal 'red' or literal 'white'
)        # end of capture no.2
\s        # space in originial, any whitespace for this one
(        # start of capture no.3
king|queen    # either literal 'king' or literal 'queen'
)        # end of capture no.3
)        # end of capture no.1
/x';

Capture (1) is useless, as it will have exactly the same contents as the=
=

whole match in (0), so lose the outside ( and ). If you don't need the  =

'red' & 'king' in your match seperately, you can use (?: to create an  =

uncaptured subpattern.

\$Pattern = '/(?:red|white) (?:king|queen)/';
-- =

Rik Wasmus

## Re: Regular Expression: How the rule is applied for the Pattern '/((red|white) (king|queen))/'

Hi Thanks for the reply,

Can you please tell me what I have under stood is right?

After the start of capture no.2 the pattern starts with the word
r is matched with t (the red king) -> NO Match
r is matched with h -> No Match
r is matched with e -> No Match
.
.
.
r is matched with r      -> Match Found
Now the engine moves to the Next character in the pattern ie e
re is matched with re    -> Match Found
red is matched with red  -> Match Found

Since we have a | it comes out from the #first capture and now we
have an empty
space

red(space) is matched with red(space)
red(space)k is matched with red(space)k
.
.
red(space)king is matched with red(space)king -> Match Found

Now the value is returned and we get the first match

array
(
[0] => the red king
)

Is the above mentioned procedure is correct?

Also I was not able to find out how the other three values

Array
(
[1] => red king
[2] => red
[3] => king
)

are matched.

Moses

## Re: Regular Expression: How the rule is applied for the Pattern '/((red|white) (king|queen))/'

On Tue, 22 Jan 2008 15:41:34 +0100, mosesdinakaran@gmail.com  =

Thank me by not top-posting :).

Not exactly how it happens 'under water', but as far as the outcome, yes=
,  =

that's correct. I take it the internals of the regex engine are not the =
=

concern, but the use of regular expressions themselves?

Euhm, I even numbered the captures for you in my previous post... Suffic=
e  =

to see every subpattern between () gets its own entry in the match array=
,  =

unless you specifiy an uncaptured subpattern (?:
-- =

Rik Wasmus

## Re: Regular Expression: How the rule is applied for the Pattern '/((red|white) (king|queen))/'

Greetings, mosesdinakaran@gmail.com.
In reply to Your message dated Tuesday, January 22, 2008, 17:41:34,

Hi and no thanks for the top-posting.

Actually, the pattern is started to match "the red king" with "(red|white)"
(mean, two matches, one with "red" and one with "white") and advance by
character till string will be matched, "the red king" -> "he red king" ->
"e red king" -> "<space>red king" -> "red king" -> Match (red) found!
As far as we have this match in brackets, we saving the (red) as #1 match with
intention to return it as 1st substring.

Nop, as far as we have captured (red) we start the mew roundtrip.
"<space>king" will be matched against space in the pattern -> it matches, so
we saving it as #2 match (no return) and start third trip.

"king" will be matched against "(king|queen)" and it matches.
Let's save it as #3 match and remember to return it as 2nd subpattern
There is no more atoms in pattern, so let's look to see what we need else.
Oh, we have an outer brackets contains both subpatterns.
So we storing #1+#2+#3 as [1] and shifting numbers of #1 and #3.

Yup, the [0] subpattern is the whole string where the match occured.

As described.

Array
(
[0] => 'the red king' // Whole string where match occured
[1] => 'red king' // The 1st subpattern ((red)<space>(king))
[2] => 'red' // The 2nd subpattern (red)
[3] => 'king' // The 3rd subpattern (king)
)

--