Sprache -- Cannot recognise this sequence

Question

I want to match strings where the first character is a letter, then it is followed by multiple characters which are either digits or letters, then finally ends with a letter. For example a11a11a is correct but a11aa11 is incorrect because it ends with a digit and not a letter.

I wrote the following code to do it:

var grammar =
    from first in Parse.Letter.Once()
    from rest in Parse.LetterOrDigit.Many()
    from end in Parse.Letter.Once()
    select new string(first.Concat(rest).Concat(end).ToArray());

var result = grammar.TryParse("a111a");

Unfortunately LetterOrDigit.Many() consumes the last letter too.

Any way to avoid this?

Answer 1

Here is a solution:

Parser<IEnumerable<char>> A = null, B = null, C = null;

var letter = Parse.Letter.Once();
var digit = Parse.Digit.Once();

B =
    (
    from d in digit
    from cs in Parse.Ref(() => C)
    select d.Concat(cs)
    ).Or
    (
        from l in letter
        from bs in Parse.Ref(() => B)
        select l.Concat(bs)
    ).Or(letter);

C = (
    from d in digit
    from bs in Parse.Ref(() => B)
    select d.Concat(bs)
    ).Or(letter);

A = (
    from l in letter
    from bs in Parse.Ref(() => B)
    select l.Concat(bs)
    ).Or(letter);

var grammar =
    from _ in Parse.WhiteSpace.Many()
    from a in A
    from __ in Parse.WhiteSpace.Many()
    select a;

The clauses in the Or 's need to be in the correct order.

A commenter recommended the use of Regular Expressions. You can use them within Sprache:

Parse.Regex("[a-z]([a-z0-9]*[a-z])?")

Answer 2

An alternative Solution:

       var text = "a11a11a";
       var n = text.Length;
       var grammer =
           from open in Parse.Letter.Once()
           from content in Parse.LetterOrDigit.Repeat(n - 2)
           from close in Parse.Letter.Once()
           select open.Concat(content).Concat(close);       

Try it