a simple substitution question

d4vis has asked for the wisdom of the Perl Monks concerning the following question:

Hullo,
I've been reading an online tutorial that illustrates a simple substitution of "them" for "us" in the following string:

$_='Us ? The bus usually waits for us, unless the driver forgets us.';
[download]

The tutorial's solution is the following:

$_='Us ? The bus usually waits for us, unless the driver forgets us.';
print "$_\n";
s/\b([Uu])s(\W)/chr(ord($1)-1).hem.$2/eg;
print "$_\n";
[download]

This works, but when I got to thinking about it...

$_='Us? The bus usually waits for us, unless the driver forgets us.';
print "$_\n";
s/\bus\b/them/ig;
print "$_\n";
[download]

also works and seems simpler to a newbie like me. I guess I'm just wondering if there's something about substitutions that I don't know yet which makes the simpler solution 'bad practice' or sloppy programming? I get the sense that the tutorial chose the solution it did based on something I don't yet know about regex's, but I can't figure out what that is.
Any guidance greatly apreciated.
-d4vis
#!/usr/bin/fnord

Comment on a simple substitution question Select or Download Code

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(Ovid) Re: a simple substitution question by Ovid (Cardinal) on Aug 21, 2000 at 22:31 UTC
tilly: The author of the tutorial may have been trying to show off, but his/her answer is more correct than the answer that d4vis provided. What happens with the following? `'Who\'s going to the store?" "Us," she replied."'` Yeah, I know. "She" is using pitiful grammar, but it's all I could come up with on the spur of the moment. Here, you lose capitalization with d4vis's regex. The unknown author came up with a nifty trick to preserve capitalization. However, it's not very clear. I'm also wondering if some versions of `locale` might break it depending upon the alphabet used. Instead, I'd use something like the following (which I feel is more clear -- but untested): `s/\b([Uu])s\b/$1 eq 'U' ? Them : them/eg;` [download] Cheers, Ovid Update: Oy! That's what I get for untested code. I guess tilly and I will send the rest of the day spanking each other (figuratively speaking). Here's the correct version of the regex (which I still think is clearer than tilly's solution): `$test =~ s/\b([Uu])s\b/$1 eq 'U' ? "Them" : "them"/eg;` [download] Tilly's solution, however, is better with multiple substitutions.	[reply] [d/l] [select]
RE: (tilly) 2: a simple substitution question by tilly (Archbishop) on Aug 21, 2000 at 22:39 UTC
D'oh! Actually the author was even more clever and figured out how to leave 'US' alone. Your solution breaks under strict though. Try this: `my %tr = qw(U T u t); s/\b([Uu])s\b/$tr{$1}hem/g;` [download] BTW I would wonder about the author's solution working with Unicode...	[reply] [d/l]
RE (tilly) 1: a simple substitution question by tilly (Archbishop) on Aug 21, 2000 at 22:19 UTC
Looks to me like the person writing the tutorial was just trying to show off. I would write it exactly like you did. To my eyes that is simpler, clearer, and should execute faster as well. Did I mention more reliable? Try the string, "What should happen with a trailing us"?	[reply]