regex, how look behind work

Look behind doesn't take space either

Look this example

(?m)(?<=a)bc.(?<=a)bc

it will only match abcabc

the matched string is bcabc

how to read this regex?

Remove the look behind first

it becomes bc.bc

OK, this will match quite a few string, like dbcebc

but, remember we have look behind, that means, before first bc, it must be an a, before second bc, it must be another a there. so....

only abcabc will match.

Look behind add another layer of restriction. easy?

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Feng