The difference is that you have to remember that in:
my $ua_name = $1 if ($ua=~ m/(opera|netscape|gecko|msie)/i);

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The whole regex is checked at every position in your string before moving on to the next position. The whole regex is checked at the current position in the string, before moving on to the next position. So at every position in the string it would check first to see if it can match opera, failing that it THEN checks to see if it can match netscape, failing that it THEN checks to see if it can match gecko, and finally failing everything else it checks to see if it can match msie. If all this fails it moves to the next position in the string and checks again, and again and again until the regex matches. (remember left most first match wins)

In the second piece of code:

my $ua_name = $1 if ( ($ua=~ m/(opera)/i) || ($ua=~ m/(netscape)/i) ||
+ ($ua=~ m/(gecko)/i) || ($ua=~ m/(msie)/i) );

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What happens in this is that it will check the whole string and for something (ie. netscape, gecko, etc..) and then move on to the next regex if it fails. I can elaborate if need be.

update: perhaps an example would help better explain how the regex engine is working, at the command line try this one-liner:

perl -Mre=debug -le "$s = 'a man a plan a camel';print $1 if $s =~ /(c
+amel|plan|monkey)/;"

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Also I struck out the first sentence which was causing confusion, and replaced it with a new first sentence. Thanks Zero Flop for pointing out my poor wording. -enlil

In reply to Re: String order in regex match - left to right, or right to left? by Enlil
in thread String order in regex match - left to right, or right to left? by Alex

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