preg_replace doubt

What I'm doing wrong ?

echo preg_开发者_Go百科replace('#\d{3}\d{3}\d{3}\d{2}#', '$1.$2.$3-$4', '12345678901');

Output would be this: 123.456.789-01. Isn't formating the string!!

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<?php
echo preg_replace('#(\d{3})(\d{3})(\d{3})(\d{2})#', '$1.$2.$3-$4', '12345678901');

is correct, because dollar sign + integer refers to content in () brackets (grouping)

demo

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You have not grouped correctly (missing brackets):

echo preg_replace('#(\d{3})(\d{3})(\d{3})(\d{2})#', '$1.$2.$3-$4', '12345678901');