TODO: Add support for numbered repetitions i.e. {n,m}, {n,}, {,m}

[eriknyquist]

Should look like this:

a{n,m} : Match n-m (inclusive) repetitions of a.
a{n,} : Match n or more (inclusive) repetitions of a.
a{,m} : Match m or less (inclusive) repetitions of a.

This can be done (I think) by adding just one new VM instruction, "jlt" or "Jump if Less Than".
This instruction takes two parameters; the index of an instruction in the VM program (jump location), and a count value. For example:

jlt 5 6

The first parameter "5" is the jump location. The second parameter "6" is the count value. The VM must keep count of how many times this instruction is executed within the containing program; if that count is less than the jlt instruction's count value, then execution will jump to the specified location, and if both values are equal then the VM will stop keeping count and execution will carry on to the next instruction as normal. The effect, in this example, is that for the first 6 times this instruction is executed it will cause execution to jump to the instruction at "5", and every time after that it will have no effect and execution will proceed normally.

These can be used with branch instructions to build the desired functionality. Some examples:

abc{n,m}

0 char a
1 char b
2 jlt 4 n
3 branch 4 6
4 char c
5 jlt 2 m+1
6 match

abc{n,}

0 char a
1 char b
2 char c
3 jlt 2 n+1
4 branch 2 5
5 match

abc{,m}

0 char a
1 char b
2 branch 3 5
3 char c
4 jlt 2 m+1
5 match

abc{n}

0 char a
1 char b
2 char c
3 jlt 2 n+1
4 match