Right you are! Fixed below.

/
   ^(?![ ])        # Doesn't start with a space
   .*?             # Anything
   (?<![a-z0-9])   # For the {5,12} to be meaningful.
   [a-z0-9]{5,12}  # Has a 5..12 lowercases or digits in a row
   (?![a-z0-9])    # For the {5,12} to be meaningful.
   .*              # Anything
   (?<![ ])$       # Doesn't end with a space
/x
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