Suzhou numerals (蘇州碼子; also 花碼) are Chinese decimal numerals:

0 〇

1 〡 一

2 〢 二

3 〣 三

4 〤

5 〥

6 〦

7 〧

8 〨

9 〩

They pretty much work like Arabic numerals, except that when there are consecutive digits belonging to the set {1, 2, 3}, the digits alternate between vertical stroke notation {〡,〢,〣} and horizontal stroke notation {一,二,三} to avoid ambiguity. The first digit of such a consecutive group is always written with vertical stroke notation.

The task is to convert a positive integer into Suzhou numerals.

Test cases

1          〡

11         〡一

25         〢〥

50         〥〇

99         〩〩

111        〡一〡

511        〥〡一

2018       〢〇〡〨

123321     〡二〣三〢一

1234321    〡二〣〤〣二〡

9876543210 〩〨〧〦〥〤〣二〡〇

Shortest code in bytes wins.

Retina, 46 bytes

/[1-3]{2}|./_T`d`〇〡-〩`^.

T`123`一二三

Try it online! Link includes test cases. Explanation:

/[1-3]{2}|./

Match either two digits 1-3 or any other digit.

_T`d`〇〡-〩`^.

Replace the first character of each match with its Suzhou.

T`123`一二三

Replace any remaining digits with horizontal Suzhou.

51 bytes in Retina 0.8.2:

M!`[1-3]{2}|.

mT`d`〇〡-〩`^.

T`¶123`_一二三

Try it online! Link includes test cases. Explanation:

M!`[1-3]{2}|.

Split the input into individual digits or pairs of digits if they are both 1-3.

mT`d`〇〡-〩`^.

Replace the first character of each line with its Suzhou.

T`¶123`_一二三

Join the lines back together and replace any remaining digits with horizontal Suzhou.

Perl 6 -p, 85 61 bytes

-13 bytes thanks to Jo King

s:g[(1|2|3)<((1|2|3)]=chr $/+57;tr/0..</〇〡..〩一二三/

Try it online!

Perl 5 -p -Mutf8, 53 bytes

s/[123]K[123]/chr$&+57/ge;y/0-</〇〡-〩一二三/

Try it online!

JavaScript (ES6), 95 bytes

Takes input as a string.

s=>s.replace(i=/./g,c=>'〡〢〣〇一二三〤〥〦〧〨〩'[c--?c<3?c|(i=!i*4):i=c+4:i=3])

Try it online!

Note

Within the callback of replace(), the variable $c$ is a 1-character string, which is always truthy (even for "0") and cannot be used as-is for arithmetic addition.

We could use +c each time we need its decimal value, but that would be a bit long.

Instead of that, we use the fact that c-- yields the value of $c$ before it's decremented but already coerced to a number.

With this single expression, we can simultaneously:

• coerce $c$ to a number


• test if it was originally set to "0"
  


• store $c-1$ in $c$