taleinat wrote:
> Gabriel Murray <gabriel.murray <at> gmail.com> writes:
>
> >
> > Hello, I'm looking for a regular expression which will match strings as
> follows: if there are symbols a, b, c and d, then any pattern is valid if it
> begins with a and ends with d and proceeds in order through the symbols.
> However, at any point the pattern may reset to an earlier position in the
> sequence and begin again from there.
> > For example, these would be valid
> patterns:aabbbaabbcccbbbcccdddaabcabcdabcdBut these would
> not:aaaaabbbbbccccaaaaadddd (goes straight from a to d)aaaaaaaaaaabbbbbccc
> (does not reach d)Can anyone think of a concise way of writing this regex? The
> ones I can think of are very long and awkward.Gabriel
> >
>
> Your cirteria could be defined more simply as the following:
> * must start with an 'a' and end with a 'd'
> * an 'a' must not be followed by 'c' or 'd'
> * a 'b' must not be followed by 'd'
In fact it is so regular that the rules for the same thing with N
letters can be written with O(1) rules: E.g. for N == 26:
* must start with a and end with z
* can go to (1) same spot (2) ahead 1 spot (3) back to any previous
spot.
Does the OP really need a regex????
Give the spots numbers (say from 1 up) instead of letters and a checker
becomes trivial:
[untested]
def check(size, path):
if not path or path[0] != 1 or path[-1] != size:
return False
pos = 1
for newpos in path[1:]:
if not(1 <= newpos <= min(size, pos + 1)):
return False
pos = newpos
return True
Cheers,
John
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