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smallest match in regexp

g00dlife
New User

Mar 11, 2003, 6:32 PM

Post #1 of 4 (1362 views)

smallest match in regexp	Can't Post

I need some help on simple regexp.

Just say I have a string for example:

$str = "start start start match1 finish finish finish";

I would like to extract the smallest string that is bounded by "start" and "finish"... ie. in this case I would like to extract the string "match1".

My first guess was to use:

$pat = "start (.*?) finish";

however this matches the string "start start match1".

I know I can use the pattern:

$pat = ".*start (.*?) finish";

and get what I want, however there's a performance issue by using the greedy matcher .* at the start of the pattern. (This is not important but i'm actually tackling a problem cause by the java package Jakarta ORO... big performace issues by using combinations of greedy and non-greedy matches on large strings.)

Is there another single regular expression that will extract "match1" from the above $str when you only know it's bounded by "start" and "finish".

Any help would be greatly appreciated.

Jasmine
Administrator

Mar 11, 2003, 8:29 PM

Post #2 of 4 (1360 views)

Re: [g00dlife] smallest match in regexp [In reply to]	Can't Post

Yep. Use a negative lookahead assertion.

Code

$str =~ /(?<!start) (.*?) finish/;

For details, please read [url=http://www.perldoc.com/perl5.8.0/pod/perlretut.html#Looking-ahead-and-looking-behind]Looking Ahead and Looking Behind in perlretut.

g00dlife
New User

Mar 12, 2003, 5:17 PM

Post #3 of 4 (1354 views)

Re: [Jasmine] smallest match in regexp [In reply to]	Can't Post

Thank you Jasmine. That solution does work with my above example... but for my problem I should have been more specific with my question.

The string should have been something like:

$str = "start start no match start start match1 finish finish";

I'm after 'match1' and all I know is that it is bounded by the strings "start " and " finish" (for the above $str example).

The question rephrased is simply: Is it possible to find the last occurance of "start (.*?) finish" in any given string without using the .* greedy matcher?

Your help is very much appreciated.

jryan
User

Mar 12, 2003, 11:05 PM

Post #4 of 4 (1351 views)

Re: [g00dlife] smallest match in regexp [In reply to]	Can't Post

I'll one-up you... Here's a solution that works without the use of the inaccurate .*? token.

Code

my $string = "start start no match start start match1 finish finish"; 
 
my @matches = $string =~ 
/ 
    start 
    ( 
         s(?!tart) 
       | f(?!inish) 
       | [^sf]+ 
    ) 
    finish 
/x; 
 
print join ("\n", @matches)

Please refer to the link that Jasmine gave for more details.

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