It's consecutive doubling and adding from left to right

• the first tr doubles every digit mod 10 1->2, ... 5->0, 6->2,...
• the second tr adds a carry bit for every 5-9 -> 1
• since the next bit is appended, the carry bit is shifted to the left
• the | does a bitwise or which adds the bits ²

for example if $n == "08" and the next bit is "1", you'll get "06" | "11" == "17" and so on.

tybalt loves constructing¹ regex-automatons on strings

¹) cryptic

²) this part is a bit (sic ;) voodoo for me

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