Is the input Bl lu ur rr ry?
Retina, 14 bytes
a`.((.) \2)*.|

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Explanation
a forces the match to be on the entire string
.((.) \2)*. just means any character (.) followed by a captured group repeated 0 to many times ((...)*) of any character saved into a second captured group ((.)) followed by a space and the same character that was matched in the second capture group (\2). The final . just matches the last character
| allows the empty string to match by having an alternative match of the empty string