正则表达式正好用2替换2个反斜杠

Question

I have a string where 2 backslashes must become 3 backslashes. With all the escaping of backslashes, I can't do it and nothing's working.

For example:

• aaaaa\\aaaaa must become aaaaa\\\aaaaa
• aaaaa\\\aaaaa STAYS aaaaa\\\aaaaa (2 backslashes in a row must not be detected)

I managed this, which works in regex101.com:

(\\\\)([^\\])

so in php, it would be:

preg_replace('/(\\\\)([^\\])/', '\\\$2', $bob);

But php gives me this error message:

preg_replace(): Compilation failed: missing terminating ] for character class at offset 10

So I don't know if it's a bug of PHP, but I need another way of doing this and I can't find the solution on google.

Answer 1

Try this regex:

(?<!\x5C)\x5C{2}(?!\x5C)

Backslashes can be a real nightmare here. It's better to use the ASCII code of the backslash right into the regex.

Sample code:

define('THREE_SLASHES', preg_quote('\\\\\\'));
define('RE_TWO_SLASHES_ONLY', '~(?<!\x5c)\x5c{2}(?!\x5c)~');

$test='aaaaa\\\\aaaaa';
echo preg_replace(RE_TWO_SLASHES_ONLY, THREE_SLASHES, $test) . '<br>';

$test='aaaaa\\\\\\aaaaa';
echo preg_replace(RE_TWO_SLASHES_ONLY, THREE_SLASHES, $test) . '<br>';

Output

aaaaa\\\aaaaa
aaaaa\\\aaaaa