regular expression match and replace multiple strings

Question

I have this:

$only_left_pattern = '/[^-{]\bleft\b/';
$subject = 'body {left: 24px;}\n p {right:30px;}';
$subject = preg_replace($only_left_pattern, 'right', $subject);
echo $subject;

I need to declare a pattern for each string I want to match. If there way I can match right or left and replace according to that? Example (syntax from my head):

$right_left_pattern = '/[^-{](\bleft\b|\bright\b/)';

// if you found left replace with right, if you found right replace with left
// acrording to the order in the pattern
$subject = preg_replace($only_left_pattern, right|left, $subject);

What I'm asking if I can do something like my example with PHP or just with regular expression.

I tried Dave's solution

$pattern = array('/[^-{]\bleft\b/','/[^-{]\bright\b/');
$replace = array('right','left');
$subject = 'body {left: 24px;} p {right: 24px;}';
$subject = preg_replace($pattern, $replace, $subject);
echo $subject;

but it doesn't work, I tested the regex not as array and it's working.

preg_replace_callback(), although you can also pass arrays to the first and second arguments of preg_replace() - but I suspect a callback is what you want. — DaveRandom
– DaveRandom, Commented Sep 24, 2012 at 11:21
thanks DaveRandom, tryed this didn't work: $pattern = array('/[^-{]\bleft\b/','/[^-{]\bright\b/'); $replace = array('right','left'); $subject = 'body {left: 24px;} p {right: 24px;}'; $subject = preg_replace($pattern, $replace, $subject); — Yehuda Hassine
– Yehuda Hassine, Commented Sep 24, 2012 at 12:56

chrislondon · Accepted Answer · 2013-06-06 17:30:51Z

The first problem is your regular expression. You have [^-{] that means that you are checking for a string that DOES NOT start with a - or a { but since {left: starts with a { the match is ignoring it. I imagine what you're trying to say is only replace the word left. To replace only part of a string you need to use back references like this:

$pattern = '/([-{]\b)(left)(\b)/';
$replace = '$1right$3';
$subject = 'body {left: 24px;} p {right: 24px;}';
$subject = preg_replace($pattern, $replace, $subject);

// result: body {right: 24px;} p {right: 24px;}
echo $subject;

Notice the parentheses in the $pattern. That is grouping those parts together. Also notice the $n in the $replace. The first parenthesis group ([-{]\b) is $1, the second (left) is $2, and the third (\b) is $3.

Now that solves replacing the left -> right but you can't do the array as suggested because this will happen:

body {left: 24px;} p {right: 24px;}

Step 1: replace left pattern
body {right: 24px;} p {right: 24px;}

Step 2: replace right pattern
body {left: 24px;} p {left: 24px;}

Oops! now you have all left's. That's not very helpful. What you really want is preg_replace_callback. preg_replace_callback will run a user defined function for each match. So what we do is we find all the lefts and rights in one go and then in our function we swap the left/right and return it plus the original string like so:

function replaceLeftRight($matches) {
    // get the opposite left/right
    $leftRight = ($matches[2] == 'left') ? 'right' : 'left';
    return $matches[1] . $leftRight . $matches[3];
}   

$pattern = '/([-{]\b)(left|right)(\b)/';
$subject = 'body {left: 24px;} p {right: 24px;}';    
$subject = preg_replace_callback($pattern, 'replaceLeftRight', $subject);

// result: body {right: 24px;} p {left: 24px;}
echo $subject;

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