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
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