php replace regular expression

Question

I need to use php to add a space between a period and the next word/letter when there's none.

For example, "This is a sentence.This is the next one." needs to become "This is a sentence. This is the next one." Notice the added space after the first period.

My problem is that even if I'm able to make a regular expression that finds every dot followed by a letter, how do I then replace that dot with a "dot + space" and keep the letter?

Also it needs to keep the case of the letter, lower or upper.

Thanks for your input.

Doing this via regex will lead to false results with things like This is a sentence."And is a quote." And this contains three dots... and all other kinds of situation where a dot is valid but a following space is wrong. You can't, in fact, parse natural language correctly with regular expressions, and even more sophisticated tools have a very hard time with that. — Tomalak
– Tomalak, Commented May 19, 2010 at 14:42
While this is true, it will at least allow me to correct some of the most obvious mistakes. — Enkay
– Enkay, Commented May 20, 2010 at 0:29

ircmaxell · Accepted Answer · 2010-05-19 15:01:44Z

9

$regex = '#\.(\w)#';
$string = preg_replace($regex, '. \1', $string);

If you want to capture more than just periods, you can do:

preg_replace('#(\.|,|\?|!)(\w)#', '\1 \2', $string);

Simply add the characters you want replaced into the first () block. Don't forget to escape special characters (http://us.php.net/manual/en/regexp.reference.meta.php)

edited May 19, 2010 at 15:01

answered May 19, 2010 at 14:32

ircmaxell

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4 Comments

Enkay Over a year ago

Thank you very much! So basically the \1 is the variable containing the letter. Do you mind explaining this or pointing me somewhere I can better understand it?

webbiedave Over a year ago

\1 is what is captured in the first group (regex contained within the first group of parentheses). It's called a back reference.

Heikki Naski Over a year ago

That fails if you have something else than a word character after the dot, like the string "Foo.-Bar"

Enkay Over a year ago

What if I want to do this for periods or commas. Now I need two back references right? How would that work? Thanks.

webbiedave · Accepted Answer · 2010-05-19 14:48:37Z

1

$str = "Will you please slow down?You're gonna kill someone.Seriously!";
echo preg_replace('/(!|\?|\.)([^\s\.\?!])/', '\1 \2', $str);

answered May 19, 2010 at 14:48

webbiedave

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3 Comments

Alan Moore Over a year ago

FYI, (!|\?|\.) can be written as [!?.], making it more efficient as well as more readable.

ircmaxell Over a year ago

@Alan, you'd need to wrap the [] in () as well for the backreference to work ([!?.])...

webbiedave Over a year ago

@Alan Moore: True. But I wanted to see if ircmaxell would copy it verbatim (as you can see, I do use brackets in the second, albeit with superfluous espaces). lol jk

Heikki Naski · Accepted Answer · 2010-05-19 14:39:54Z

0

$str = "This is a sentence.This is the next one.";
echo preg_replace("#\.(\S)#", '. \1', $str);

answered May 19, 2010 at 14:39

Heikki Naski

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2 Comments

ircmaxell Over a year ago

That will erroneously turn "this..." into "this. .."

Heikki Naski Over a year ago

@ircmaxell, it's just what the OP asked; add a period if the following character isn't a space (I used a non-whitespace, tho). There are probably a lot of other situations where it won't work either, URIs for example.

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