Decode the Dreaded Alphabet Cypher™️
JavaScript (ES9),  37 33  26 bytes
Returns an array of characters.
Either a negative look-behind:

s=&gt;s.match(/(?&lt;![b-z])./g)

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Or a positive look-behind:
s=&gt;s.match(/(?&lt;=a| |^)./g)

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25 bytes
If the input is guaranteed not to contain leading or double spaces (or we're allowed to remove and collapse them respectively), the positive look-behind can be made 1 byte shorter:

s=&gt;s.match(/(?&lt;=a|\b)./g)

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