From: Dave Burt <dave@...>
Date: 2005-11-22T01:57:24+09:00
Subject: Re: Regular Expressions and Ruby

George Lunsford <george.lunsford@gmail.com> writes:

> Hi, I'm new to the list and I hope this is the right place to ask the
> question, but I recently started learning ruby and I was writing a short
> little program to test if a word was made up of symbols from the periodic
> table. I figured I'd use ruby's built-in regex support, so I ended up
> writing a regex that looks something like this:
>  (element1|element2|element3...|elementx)+
>  Basically it's just got all 115 or so of the elements in there, and it's
> testing for one or more, right? Unfortunately this doesn't seem to work
> properly as a (nonsense) word like "presenti" should be periodic according
> to my list of elements, [p][re][se][n][ti]. Now, I can duplicate this
> behavior with the regex coach oddly enough. The problem seems to be that 
> it
> parses [p][re][s] instead of [se]. I'm wondering if anyone can explain 
> this
> behavior to me, as I'm new to both regexps and ruby and the only regex
> experience I've had previous to this is in some theory classes. Thanks. 
> The
> full expression is below.
>
> (He|Li|Be|C|O|Ne|Na|Mg|Al|Si|S|Cl|Ar|Ca|Sc|Ti|Cr|Mn|Fe|Co|Ni|Cu|Zn|Ga|Ge|As|Se|Br|Kr|Rb|Sr|Zr|Nb|Mo|Tc|Ru|Rh|Pd|Ag|Cd|In|Sn|Sb|Te|Xe|Cs|Ba|Hf|Ta|Re|Os|Ir|Pt|Au|Hg|Tl|Pb|Bi|Po|At|Rn|Fr|Ra|Rf|Db|Sg|Bh|Hs|Mt|Uun|Uuu|Uub|Uuq|Uuh|Uuo|Ds|La|Ce|Pr|Nd|Pm|Sm|Eu|Gd|Tb|Dy|Ho|Er|Tm|Yb|Lu|Ac|Th|Pa|Np|Pu|Am|Cm|Bk|Cf|Es|Fm|Md|No|Lr|H|B|N|F|P|V|Y|I|W|Uaal)+

You'll note if you match against /^(He|...|Uaal)$/i it matches the full 
string "presenti". Read perlretut for more info on how regexps work. 
Basically, it is greedy, but it won't backtrack unless it has failed to find 
a match. With your example, the S matches, then the following characters 
don't match anything, but it has already succeeded, so it doesn't bother 
trying Se.

http://www.cs.rit.edu/~afb/20013/plc/perl5/doc/perlretut.html#grouping 
things and hierarchical matching

Cheers,
Dave