gistfile1.php

<?php

$pattern = '/(?>\.no-touch\s+)?((?>[^,{;}\s][^,{;}]*?:hover)(?>[^,{;}]+)?)(?=[{,])/i';

$replacement = ".no-touch $1";

$subject = <<<CSS

a
{
    color: blue;
}

a:hover
{
    color: blue;
}

.no-touch a:hover
{
    color: blue;
}

.no-touch a:hover, b
{
    color: blue;
}

a, b:hover, c
{
    color: blue;
}

a,
b:hover,
c
{
    color: blue;
}

a
b:hover
c
{}

a{}b:hover,c d:hover e {}

CSS;

$expected = <<<CSS

a
{
    color: blue;
}

.no-touch a:hover
{
    color: blue;
}

.no-touch a:hover
{
    color: blue;
}

.no-touch a:hover, b
{
    color: blue;
}

a, .no-touch b:hover, c
{
    color: blue;
}

a,
.no-touch b:hover,
c
{
    color: blue;
}

.no-touch a
b:hover
c
{}

a{}.no-touch b:hover,.no-touch c d:hover e {}

CSS;

$result = preg_replace($pattern, $replacement, $subject);

if ($result === $expected) {
    echo "<h2>Test passes.</h2>";
} else {
    echo "<h2>Test fails.</h2>";
}

echo "<h4>Result:</h4>";
echo "<pre>$result</pre>";
echo "<hr/>";
echo "<h4>Expected:</h4>";
echo "<pre>$expected</pre>";