Should match valid UART frames in a sequence of ones and zeros

"11111010101010011111111" "10010011001100100011111" "11111010101010101111111"

The regular expressions should match once in the first test sequence, twice in the second and never in the third

The regex consists of the following parts:

0       // Match exactly the start bit
[01]{8} // Match on the eight data bits
[01]    // Match on the parity bit (withou checking it)
1       // Match exactly on the stop bit

There is no need to check for the presence of a second stop bit as we are already in the accept state

The complete solution is:

0[01]{8}[01]1

To get the actual data bits out of the regular expression we can optionally group them into a capuring group.

0([01]{8})[01]1

    ╔═══════════╗
    ║  Start    ║
    ╚════╤══════╝
         │
         ▼
    ╭────────────╮
┌─▶ │ test_start │◀────┐
│   │            ├╼ 1 ╾┘
│   ╰────┬───────╯

…

1 /*     ╭───────────╮
2  *     │ Start     │
3  *     ├┄┄┄┄┄┄┄┄┄┄┄┤
4  *     │ ped:off   │
5  *     │ car:off   │
6  *     ╰────┬──────╯
7  *          ▼        */

The FSM diagram is included as a comment in the source code

The machine is designed to be a moore machine, every state explicitly states the output values and following state

1 int isctn_step(int state_old)
2 {
3   switch(state_old) {
4   case ISCTN_START:
5     tl_car(TL_OFF);
6     tl_pedestrian(TL_OFF);
7     state_new= ISCTN_PED_GREEN;
8     break;

…

The isctn_step function is the so called transfer function, it takes the current state of the machine and outputs the following state based on the inputs and the time

A pure state machine can not use delays in the transfer function

As a little gimmick the implementation uses an inductive coil in the "road" to detect cars

The source code contains a schematic of the circuit used