Revision cb8b5c8d-c33a-4139-84e8-00381689e64f - Code Golf Stack Exchange

# [C (gcc)], 70 bytes

<!-- language-all: lang-c -->

 n;f(char*a){*a-48?n=!printf(n?"%d":"%c",n?:*a++),*a&&f(a):f(a+!!++n);}

[Try it online!][TIO-lbxu3is1]

[C (gcc)]: https://gcc.gnu.org/
[TIO-lbxu3is1]: https://tio.run/##FcnBCsIwDADQb2lgI2k2iFsV6ZB@S4hUdzCM4W347VUv7/JsfJi15ktFe@oelY6oY7oWv4VtX/1d0Qt0d8jQGQxeclRmGqL2fUWl/INDYHZaPu2lqyMdFeE0zeksMl8mEUlJ4N9f "C (gcc) – Try It Online"

# [C (gcc)], 62 bytes by c--

<!-- language-all: lang-c -->

 n;f(int*a){*a-48?n=!printf(n?"%d":a++,n),*a&&f(a):f(a+!!++n);}

[Try it online!][TIO-lgk4tnlu]

[C (gcc)]: https://gcc.gnu.org/
[TIO-lgk4tnlu]: https://tio.run/##S9ZNT07@/z/POk0jM69EK1GzWitR18TCPs9WsaAIKJKmkWevpJqiZJWora2Tp6mjlaimlqaRqGkFJLQVFbW18zSta//nJmbmaWhWp2n4KBkaGZuYGhgYmxkZGBiYmBgogeQB "C (gcc) – Try It Online"