Re: [Boost-users] [regex] difficulties with the "Leftmost Longest" Rule

18 Dec 2006

      Detlef Meyer-Eltz wrote:
...
I have a difficulty to predict, which part of a regular expression
will match.
Example:
I have a regular expression for a general HTML tag: <[^>]*>
combined with an expression for the body tag: ]*)>
to: (<[^>]*>)|(]*)>)
This expression matches the text: <body bgcolor="white">
As both alternatives can match the input with the same length, I
expected, that the repeated fouth part of the "Leftmost Longest" Rule
would determine, which alternatve is chosen:
4. Find the match which has matched the first sub-expression in the
  leftmost position, along with any ties.  If there is only on(e)
  such match possible then return it.
// note the missing 'e'
As the tag-expression has no sub-expression at all, the
body-expression should win. Its sub-expression could match, but
doesn't. It seems to me, that the sequence of the alternatives
determines the match.
Now I guess, that I misinterpreted 4.: its not a means to predict the
matching alternative but only to find the one that matched
accidentally? My software constructs lexers from elementary
expressions automatically. So it's important for me to direct and
predict the expected matching alternative. Are there any other rules?
Does the sequence of the alternatives determine the match
unmistakably?
Which Boost.Regex version are you using, and how are you compiling the 
expression?

Recent versions default to the Perl matching rules: *which do not use the 
leftmost longest rule*.  They match based on a "first match found" rule, so 
if the first alternative leads to a match then subsequent alternatives are 
never examined.

If you really want leftmost-longest semantics, then compile the expression 
as a POSIX extended regex, but of course then you loose the ability to use 
Perl-like regex extensions.

HTH, John.

PS, your analysis of the leftmost-longest rule looks correct however.

Re: [Boost-users] [regex] difficulties with the "Leftmost Longest" Rule

John Maddock