PHP Заменить определенные вхождения в строке

Вот рабочий пример, из которого вы можете поучиться ...

<?php 

// The haystack
$haystack1  = " Wireless sensor networks (WSNs) enable new applications and 
require\r\nnon-conventional paradigms for protocol design due to several 
constraints. Owing to the\r\nrequirement for low device complexity together 
with low energy consumption (i.e., long\r\nnetwork lifetime), a proper 
balance between communication and signal/data processing\r\ncapabilities 
must be found. This motivates a huge effort in research activities, 
standardization\r\nprocess, and industrial investments on this field since 
the last decade.\r\n This survey paper aims\r\nat reporting an overview of 
WSNs technologies, main applications and standards, features\r\nin WSNs 
design, and evolutions. In particular, some peculiar applications, such as 
those\r\nbased on environmental monitoring, are discussed and design 
strategies highlighted; a case\r\nstudy based on a real implementation is 
also reported. Trends and possible evolutions are\r\ntraced. Emphasis is 
given to the IEEE 802.15.4 technology, which enables many applications\r\nof 
WSNs. Some example of performance characteristics of 802.15.4-based networks 
are\r\nshown and discussed as a function of the size of the WSN and the data 
type to be exchanged\r\namong nodes. ";

// The first needle
$needle1 = ".\r\n";
// The second needle
$needle2 = "\r\n";


// First we find all occurrances of .\r\n and replace it with a ramdom_text_of_your_choice.
$results1 = ReplaceText1($haystack1,$needle1);



// We are now free to remove only the \r\n instances...
$results2 = ReplaceText2($haystack1,$needle2);



// Finally we change ramdom_text_of_your_choice back to .\r\n
$results3 = ReplaceText3($haystack1,$needle1);

echo $results3;



// The function
function ReplaceText1($haystack1,$needle1){

return str_replace($needle1, 'ramdom_text_of_your_choice', $haystack1);

}

// The function
function ReplaceText2($haystack1,$needle2){

return str_replace($needle2, '', $haystack1);

}

// The function
function ReplaceText3($haystack1,$needle1){

return str_replace($needle1, '.\r\n', $haystack1);

}


?>

Поскольку str_replace заменяет символы слева направо, вы можете использовать это хитрое решение:

echo str_replace([".\r\n", "\r\n", '__xyz__'],['__xyz__', '', ".\r\n"],$text);
// first replace `.\r\n` with some unique list sequence `__xyz__`
// then replace the remaining `\r\n`
// then replace unique list sequence `__xyz__` back with `.\r\n`

Решение Regexp находится здесь:

echo preg_replace("/([^\.])\r\n/", '$1', $text);

Вы можете использовать отрицательный просмотр назад в регулярном выражении:

$text = preg_replace('~(?<!\.)\r\n~', ' ', $text);

Пример :

$text = "foo\r\nbar.\r\ntest";
$text = preg_replace('~(?<!\.)\r\n~', ' ', $text);
echo $text; // "foo bar.\r\ntest"