How to replace one or two consecutive line breaks in a string?

Question

I\'m developing a single serving site in PHP that simply displays messages that are posted by visitors (ideally surrounding the topic of the website). Anyone can post up to

Answer 1

\R is the system-agnostic escape sequence which will match \n, \r and \r\n.

Because you want to greedily match 1 or 2 consecutive newlines, you will need to use a limiting quantifier {1,2}.

Code: (Demo)

$string = "Porcupines\nare\n\n\n\nporcupiney.";

echo preg_replace('~\R{1,2}~', '
', $string);

Output:

Porcupines
are

porcupiney.

---

Now, to clarify why/where the other answers are incorrect...

@DavidZ's unexplained answer fails to replace the lone newline character (Demo of failure) because of the incorrect quantifier expression.

It generates:

Porcupines\nare

porcupiney.

The exact same result can be generated by @chaos's code-only answer (Demo of failure). Not only is the regular expression long-winded and incorrectly implementing the quantifier logic, it is also adding the s pattern modifier.

The s pattern modifier only has an effect on the regular expression if there is a dot metacharacter in the pattern. Because there is no . in the pattern, the modifier is useless and is teaching researchers meaningless/incorrect coding practices.