Notations to reference captures in the replacement string:
<?php
// (1) \\number notation: (1 to 9, not greater than 9)
echo mb_ereg_replace('(\S*) (\S*) (\S*)', '\\1 jam, \\2 juice, \\3 squash', 'apple orange lemon').'<br>'; // apple jam, orange juice, lemon squash
// (2) \k<number> notation: (also greater than 9) (also as \k'number')
echo mb_ereg_replace('(\S*) (\S*) (\S*)', '\k<1> jam, \k<2> juice, \k<3> squash', 'apple orange lemon').'<br>'; // (same as above)
// (3) \k<word> notation: (also as \k'word')
echo mb_ereg_replace('(?<word1>\S*) (?<word2>\S*) (?<word3>\S*)', '\k<word1> jam, \k<word2> juice, \k<word3> squash', 'apple orange lemon').'<br>'; // (same as above)
// Note non-named-subpatterns like "(\S*)" should not be used with named-subpatterns like "(?<word>..)" because non-named-subpatterns cannot be captured when named-subpatterns exist.