pattern matching in perl

ludeKing · 04-01-2004, 08:11 AM

Hi all,
Trying to get this prog to match exactly 3 vowels in a string. Here is my code but it doesn't wrok and should! Read 'perldoc perlre' and it says to use what I ahve used for matching 'at least n times but no more than m times, where n and m are like so : {n, m}.

heres the code:

Code:

#!/usr/bin/perl -w
use strict;

# a program to test for a sequence of at least 3 vowels

# declare variables
my $string;


print("\n\nThis program will check to see if your word contains\n");
print("a sequence of exactly 3 vowels.\n");
print("The program will tell you if there is a match or not!\n");
print("You may continue entering single words to test until CTRL-D.\n\n");

while ($string = <STDIN> ){
    chomp ($string);

if ($string =~ m/[aeiou]{3,3}?/) {             
    print ("We have a match\n\n");
} else {
    print ("Nope!\n\n");
}
}

it should be matching. hmm. any ideas???

jpbarto · 04-01-2004, 09:02 AM

try:
if ( $string=~m/.*[aeiouAEIOU]{3}.*/ )

the regular expression allows for any number of characters to come before the 3 vowel sequence ('.*') and then allows for three vowels upper or lower case and then any number of trailing characters ('.*').

Hope that helps,
jpbarto

ssba · 04-01-2004, 12:15 PM

Perhaps you could try something like the following? (NOTE: Untested)

Code:

#!/usr/bin/perl -w
use strict;

# a program to test for a sequence of at least 3 vowels

# declare variables
my $string;


print("\n\nThis program will check to see if your word contains\n");
print("a sequence of exactly 3 vowels.\n");
print("The program will tell you if there is a match or not!\n");
print("You may continue entering single words to test until CTRL-D.\n\n");

while (<STDIN> ){
    $string = chomp;

if ($string =~ /[aeiou]{3}/) {             
    print ("We have a match\n\n");
} else {
    print ("Nope!\n\n");
}
}

You can always do {3} instead of {3,3} -- it tells the parser to find *exactly* three.

I'm not sure if this will work or not, however, as I have not tested it.

mgatny · 04-01-2004, 04:07 PM

$string =~ m/(^|[^AEIOUaeiou])([AEIOUaeiou]{3})($|[^AEIOUaeiou])/

aluser · 04-01-2004, 06:38 PM

Code:

echo -e 'foo\nbar\nbaaaz'|perl -ne 'print if /[aeiou]{3}/'

this prints just "baaaz". This is exactly what ssba has. It won't print "babaaz", so if you want that you need something different.

You don't need .* because perl accepts a match anywhere in the string: the match is not anchored to the ends unless you do so manually with ^ and $

What bothers me here is that the original poster's pattern seems to work fine: I don't understand what the '?' is doing.

ludeKing · 04-01-2004, 10:43 PM

The ? is to make it a minimal match. For some strange reason, the original code DOES NOT WORK.

ie it will not match on 2 vowels, it WILL match on 3 vowels, but it wil lalso match on 4 vowels!!!

ie:

Code:

ae
Nope!

aei
We have a match

aeio
We have a match

aeiou
We have a match

aaa
We have a match

aa
Nope!

AAAAARRRRGGHHH!

and that is with {3}?
also the same for just {3}
and for {3,3}.

Strike · 04-02-2004, 12:57 AM

Just for fun, here's a working python version:

Code:

#!/usr/bin/env python
 
import re
import sys
 
line = sys.stdin.readline()
while line:
    if re.search('[aeiou]{3}', line, re.IGNORECASE):
        print "Match!"
    else:
        print "No match!"
    line = sys.stdin.readline()

ludeKing · 04-02-2004, 01:04 AM

Dammit, why does your work with the {3} but mine doesn't????

aluser · 04-02-2004, 09:51 AM

Quote:

it will not match on 2 vowels, it WILL match on 3 vowels, but it wil lalso match on 4 vowels!!!

I should have seen that coming

A sequence of 4 vowels does contain a sequence of 3, so it matches on the first 3. (A clearer definition of "does not work" might have helped someone answer sooner)

You can either do something like /(^|[^aeiou])[aeiou]{3}([^aeiou]|$)/, recognizing that the two adjacent characters may be included in $&, or you can use negative lookahead and lookbehind assertions. I don't remember the syntax for those so look them up

This second solution will be cleaner.

aluser · 04-02-2004, 09:53 AM

I should mention that (?: ...) does the same as ( ...) as long as you aren't using the $1, $2 etc variables afterwards, and is faster. It's also less clear :/