Re: Regular Expressions

On Tuesday 12 Oct 2004 5:56 am, Torben Ęgidius Mogensen wrote:
> A regular expression can not
> handle arbitray nesting depts, so you would either need to use a
> counter in the action of the regular expression or limit yourself to a
> fixed limit on the number of nested tables and write a regular
> expression for each level of nesting.


If you are using perl, there is another option for solving the nested
parentheses problem, which is to write a recursive regular expression:


my $np;
$np = qr{
                  \(
                      (?: # Non-capture group
                            (?> [^( )]+ ) # Non-capture group w/o backtracking
                        |
                            (??{ $np }) # Group with matching parens
                      )*
                  \)
                }x;


This generates a rexexp $np which matches:
'(' followed by zero or more iterations of:
      either a string with no parentheses
      or something which matches $np,
followed by ')'.


In effect you have a lazy description of an infinite regexp: so no counter
is needed, nor is there a limit on the depth of parentheses.


--
Martin


Martin.Ward@durham.ac.uk http://www.cse.dmu.ac.uk/~mward/ Erdos number: 4
G.K.Chesterton web site: http://www.cse.dmu.ac.uk/~mward/gkc/

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