PHP Regex - How to do str_replace on preg_replace numeric backreferences?

Question

I had the following PHP Regex code, to replace all emails with a "[removed email]" replacement string; and it worked nicely:

$pattern = "/[^@\s]*@[^@\s]*\.[^@\s]*/"; 
$replacement = "[removed email]";
$body_highlighted = preg_replace($pattern, $replacement, $body_highlighted);

However, the email replacement strategy changed and now I need to actually show the emails but replace certain parts of it. I wanted to use str_replace on the numeric backreferences, like this, but it doesn't work.

$pattern = "/[^@\s]*@[^@\s]*\.[^@\s]*/"; 
$email_part = "$0";
$replacement = str_replace('a','b', $email_part); // replace all letter A with B in each email
$body_highlighted = preg_replace($pattern, $replacement, $body_highlighted);

Any idea what I am doing wrong?

str_replace runs at calling, not as a binding. There is no a in $0 so it remains unchanged. Likely you want to be using preg_replace_callback. — user3783243
– user3783243, Commented Sep 17, 2020 at 2:24

FluffyKitten · Accepted Answer · 2020-09-17 02:47:28Z

1

You are using str_replace on the actual string $0 and not the backreference it references, which is why it's not working.

You want to do the str_replace while you are doing the preg_replace, so you can use preg_replace_callback to use a callback function to get the "email part" and perform the string manipulation on it during the regex replacement.

To extract the first part of the email (before the '@') and change it:

$pattern = "/([^@\s]*)(@[^@\s]*\.[^@\s]*)/"; 
$body_highlighted = preg_replace_callback($pattern, 'change_email', $body_highlighted);

/* str_replace the first matching part on the email */
function change_email($matches){
  return str_replace('a','b', $matches[1]).$matches[2];
}

If you used this with, for example:
$body_highlighted = "My email is [email protected]";
the result: My email is [email protected]

Note the changes to the regex in $pattern to split the email in two parts - before @ and the part include @ and the domain name. These are accessed in the callback function as $matches[1] and $matches[2].

If you want to access the domain part of the email address (after the @):

You can split the email into 3 parts (before @, the @ and everything after the @) you can use the following:

$pattern = "/([^@\s]*)(@)([^@\s]*\.[^@\s]*)/"; 
$body_highlighted = preg_replace_callback($pattern, 'change_email', $body_highlighted);

function change_email($matches){
  return str_replace('a','b', $matches[1]).$matches[2].str_replace('y','z', $matches[3]);
}

edited Sep 17, 2020 at 2:47

answered Sep 17, 2020 at 2:42

FluffyKitten

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1 Comment

jjj Over a year ago

Thank you for your time and expertise. This is exactly what I was looking for. Much appreciated.

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