$re = '/\.eps\d+\.\d+[-_.](.+?)(?:\.720p.+)\.(\w+)$/m';
$str = 'Mr.Robot.S02E01.eps2.0_unm4sk-pt1.tc.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E02.eps2.0_unm4sk-pt2.tc.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E03.eps2.1_k3rnel-pan1c.ksd.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E04.eps2.2_init_1.asec.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E05.eps2.3.logic-b0mb.hc.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E06.eps2.4.m4ster-s1ave.aes.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E07.eps2.5_h4ndshake.sme.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E08.eps2.6.succ3ss0r.p12.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E09.eps2.7_init_5.fve.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E10.eps2.8_h1dden-pr0cess.axx.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E11.eps2.9_pyth0n-pt1.p7z.720p.WEB-DL.x264-[MULVAcoded].mkv
Mr.Robot.S02E12.eps2.9_pyth0n-pt2.p7z.720p.WEB-DL.x264-[MULVAcoded].mkv';
$subst = ".$1.$2";
$result = preg_replace($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