how to explain such a regular syntax

Its was picked from drupal source code and i don't know how does it works. This is the code : preg_match('/^\[?(?:[a-z0-9-:\]_]+\.?)+$/', $host); After followed the regular syntax ,i try to cut it into pieces to make it simpler to understand , part a1: ^\[? part a2:([a-z0-9-:\]_]+\.?)+$ Then go on cut part a2: a2_1: ?: a2_2: ([a-z0-9-:\]_]+\.?)+$ a2_3: a2_2 and +$ a1 means:start with nothing or one "?" a2_2 means character must be in a-z,0-9 ,"]","_" and the number must be only one a2_3 means there must be one and only matched a2_2 string . And i dint't got a2_1 mean. Anyone could explain the it works and the fault of my analysis are appreciated! All you best ------------------------ What we are struggling for ? The life or the life ?