正则表达式压缩新行不转换为PHP

Question

I'm trying to take markdown (generated from HTML) and strip out extra newlines that were only in the HTML for formatting purposes. I have a working example of the regex on https://regex101.com/r/dEhyN3/1 but when I convert it to PHP it's not working and I'm struggling to understand why.

public function squashEmptyLineBreaks(string $markdown) : string
{
    return preg_replace('/
{2,}/', '

', $markdown);
}

Test:

    /** @test */
    public function verifySquashEmptyLineBreaks()
    {
        $original = <<<EOF
**Language**

 - Added four languages: Italian, Portuguese (Brazil), Spanish (Mexico) and Chinese (Traditional)





**Bug fixes**


 - Fixed camera jittering for passenger sitting on the back of a motorcycle with sidecar
EOF;
        $expected = <<<EOF
**Language**

 - Added four languages: Italian, Portuguese (Brazil), Spanish (Mexico) and Chinese (Traditional)

**Bug fixes**

 - Fixed camera jittering for passenger sitting on the back of a motorcycle with sidecar
EOF;

        $this->assertEquals($expected, $this->parsesMarkdown->headersNeverAtEndOfLine($original));
    }

This test continues to fail and the output is unmodified. Am I crazy or am I overlooking something?

Answer 1

Beware, in single quoted strings, sequences such as are not processed. Use double-quoted strings instead.

public function squashEmptyLineBreaks(string $markdown) : string
{
    return preg_replace("/
{2,}/", "

", $markdown);
}