$re = '/\h*(?<!\=|\!|^)([!=<>+-]+)(?!\=|\!|$)\h*/m';
$str = 'а= 3
а =3
а=3
а= 3
а =3
а=3
а= 3
а =3
а=3
a+b
a+ b
a +b
a + b
c>d
c >d
c> d
a<b
a <b
a < b
a< b
a==b
a<>b
a !=b
========================';
$subst = " \1 ";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php