• It reads STDIN line by line
• substitutes ; by ' if it resides within a word and next character is a 'm' or 'n' or 's' or 't'
• writes result to STDOUT

e.g. input: don;t output: don't

perl -MO=Deparse -wpe "s/\b;([mnst])/'$1/g"

BEGIN { $^W = 1; }
LINE: while (defined($_ = <ARGV>)) {
    s/\b;([mnst])/'$1/g;
}
continue {
    print $_;
}

-e syntax OK
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Weird...

$ perl -MO=Deparse -wpe "s/\b;([mnst])/'$1/g"
BEGIN { $^W = 1; }
LINE: while (defined($_ = <ARGV>)) {
    s/\b;([mnst])/'/g;
}
continue {
    print $_;
}
-e syntax OK
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Where did my $1 went to? :-|

This should be something trivial, but... my $1 is disappearing... :-| I'm going from "don;t" to "don'" :-|

Wasn't ' the same thing as ::?

Your shell has interpolated $1 before it came to perl

You may try perl -MO=Deparse -wpe 's/\b;([nmst])/\'$1/g'

On the command line, feed it various names.

See what it does to a name like "Mc;master".

As far as i can tell, it will print each line of STDIN to STDOUT with any semi-colon preceded by a word charater and followed by one of the letters m, n, s, or t converted to a single quote. I don't know why one would want to do such a thing, but that appears to be what that does.

I don't know why one would want to do such a thing

It's a common typo to write "don;t" (etc) instead of "don't". Maybe he does this a lot and is asking us to ignore any such typos?

Or maybe he's a semi-colon phobic ;-)

What part can't you figure out?

• perl -- runs perl
• -wpe "s/\b;([mnst])/'$1/g" -- passes in the following command line arguments:
  • w -- enable warnings (p.502);
  • p -- loop this script around each input line (p.499); and
  • e "s/\b;([mnst])/'$1/g" -- some search and replace regular expression.
    • This regexp takes an input line and looks for \b;([mnst]), captures whatever it finds inside the brackets, and replaces it with '$1, where $1 is the thing it captured. The \b is a backspace key (p.161) word boundary, capturing and clustering is covered on p.182, using square brackets to define a list of characters is covered on p.201.

So it probably allows belg4mit to clean up text files where he hit the ; instead of the ', since they're next to each other on his keyboard (as they are on mine.

Did you figure out the part that you missed now?

Update: As my learned brothers NetWallah and VenTatsu point out, \b is not a backspace in this context but a word boundary. Thanks brothers.

Update 2: Again, as DigitalKitty has pointed out, my conslusion doesn't include the corrections described in my first update. Hope fully these updates don't get longer than the original post. :(

   talexb:   The \b is a backspace key

Nope.

From "perlre":

\b Match a word boundary

    Earth first! (We'll rob the other planets later)

You have a minor problem with your explinaiton, from perlreref

This one works differently from normal strings:

   \b  An assertion, not backspace, except in a character class
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In regular expressions \b matches word boundries.