PHP preg_replace single lines, ignore multiple lines possible?

I have spent a few hours on this and cannot figure it out but due to my lack of preg_match knowledge even after studying it may be something easy. This works 开发者_如何学JAVAin removing multiple lines into one line:

$string = preg_replace('/^\n|^[\t\s]\n/m', "", $string); 

How can I reverse its function so that it ignores multiple lines and removes single lines of space? So if the input is:

one

two



three

It would remove the space between one and two, but ignore the multiple spaces before three so the result would look like:

one
two



three

Thanks.

EDIT POST

Thanks guys. 3emad, your code works for my example. I was wrong, I want this to work with code instead of just my text and when I tried using

// ----- limits -----



$new_limit = 7;       // maximum days in What's New section

$hot_limit = 20;      // top 20 most accessed links

$toprated_limit = 20; // top 20 most rated links 



// ----- bold the keyword in search result -----



$bold_keyword = 1;

it didn't remove the single line between the variables while ignoring the double space leading to the comments.

---

Here is the regular expression:

[\r|\r\n](?=\w)

This works for your example, you can test it at: RegExr

the secret was to use a positive look ahead matching, you can learn more about "look ahead" here

$str = '// ----- limits -----



$new_limit = 7;       // maximum days in What\'s New section

$hot_limit = 20;      // top 20 most accessed links

$toprated_limit = 20; // top 20 most rated links 



// ----- bold the keyword in search result -----



$bold_keyword = 1;';

echo '<pre>';
echo preg_replace('/\r?\n(?=[\w|\$])/m','',$str);
echo '</pre>';

Helpful PHP reference for regular expressions

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Not sure this is perfect solution for you, this will remove extra line + extra space + tabs

<?php
$input="this is  nimit

and friends
    this must be in single line.

@234 234234
                I have test this.
";
$input = str_replace(' ','_',$input);
$result = preg_replace('/\s+/','<br>',$input);
$result = str_replace('_',' ',$result);
echo $result;
?>