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PERLFAQ6(1)
NAME
perlfaq6 - Regular Expressions ($Revision: 1.20 $, $Date: 2003/01/03
20:05:28 $)
DESCRIPTION
This section is surprisingly small because the rest of the FAQ is littered
with answers involving regular expressions. For example, decoding a URL
and checking whether something is a number are handled with regular
expressions, but those answers are found elsewhere in this document (in
perlfaq9: ``How do I decode or create those %-encodings on the web'' and
perlfaq4: ``How do I determine whether a scalar is a
number/whole/integer/float'', to be precise).
How can I hope to use regular expressions without creating illegible and
unmaintainable code?
Three techniques can make regular expressions maintainable and
understandable.
Comments Outside the Regex
Describe what you're doing and how you're doing it, using normal Perl
comments.
# turn the line into the first word, a colon, and the
# number of characters on the rest of the line
s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;
Comments Inside the Regex
The "/x" modifier causes whitespace to be ignored in a regex pattern
(except in a character class), and also allows you to use normal
comments there, too. As you can imagine, whitespace and comments help
a lot.
"/x" lets you turn this:
s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;
into this:
s{ < # opening angle bracket
(?: # Non-backreffing grouping paren
[^>'"] * # 0 or more things that are neither > nor ' nor "
| # or else
".*?" # a section between double quotes (stingy match)
| # or else
'.*?' # a section between single quotes (stingy match)
) + # all occurring one or more times
> # closing angle bracket
}{}gsx; # replace with nothing, i.e. delete
It's still not quite so clear as prose, but it is very useful for
describing the meaning of each part of the pattern.
Different Delimiters
While we normally think of patterns as being delimited with "/"
characters, they can be delimited by almost any character. perlre
describes this. For example, the "s///" above uses braces as
delimiters. Selecting another delimiter can avoid quoting the
delimiter within the pattern:
s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
s#/usr/local#/usr/share#g; # better
I'm having trouble matching over more than one line. What's wrong?
Either you don't have more than one line in the string you're looking at
(probably), or else you aren't using the correct modifier(s) on your
pattern (possibly).
There are many ways to get multiline data into a string. If you want it to
happen automatically while reading input, you'll want to set $/ (probably
to '' for paragraphs or "undef" for the whole file) to allow you to read
more than one line at a time.
Read perlre to help you decide which of "/s" and "/m" (or both) you might
want to use: "/s" allows dot to include newline, and "/m" allows caret and
dollar to match next to a newline, not just at the end of the string. You
do need to make sure that you've actually got a multiline string in there.
For example, this program detects duplicate words, even when they span line
breaks (but not paragraph ones). For this example, we don't need "/s"
because we aren't using dot in a regular expression that we want to cross
line boundaries. Neither do we need "/m" because we aren't wanting caret
or dollar to match at any point inside the record next to newlines. But
it's imperative that $/ be set to something other than the default, or else
we won't actually ever have a multiline record read in.
$/ = ''; # read in more whole paragraph, not just one line
while ( <> ) {
while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha
print "Duplicate $1 at paragraph $.\n";
}
}
Here's code that finds sentences that begin with "From " (which would be
mangled by many mailers):
$/ = ''; # read in more whole paragraph, not just one line
while ( <> ) {
while ( /^From /gm ) { # /m makes ^ match next to \n
print "leading from in paragraph $.\n";
}
}
Here's code that finds everything between START and END in a paragraph:
undef $/; # read in whole file, not just one line or paragraph
while ( <> ) {
while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries
print "$1\n";
}
}
How can I pull out lines between two patterns that are themselves on
different lines?
You can use Perl's somewhat exotic ".." operator (documented in perlop):
perl -ne 'print if /START/ .. /END/' file1 file2 ...
If you wanted text and not lines, you would use
perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...
But if you want nested occurrences of "START" through "END", you'll run up
against the problem described in the question in this section on matching
balanced text.
Here's another example of using "..":
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof();
# now choose between them
} continue {
reset if eof(); # fix $.
}
I put a regular expression into $/ but it didn't work. What's wrong?
Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, but
don't get your hopes up. Until then, you can use these examples if you
really need to do this.
Use the four argument form of sysread to continually add to a buffer.
After you add to the buffer, you check if you have a complete line (using
your regular expression).
local $_ = "";
while( sysread FH, $_, 8192, length ) {
while( s/^((?s).*?)your_pattern/ ) {
my $record = $1;
# do stuff here.
}
}
You can do the same thing with foreach and a match using the
c flag and the \G anchor, if you do not mind your entire file
being in memory at the end.
local $_ = "";
while( sysread FH, $_, 8192, length ) {
foreach my $record ( m/\G((?s).*?)your_pattern/gc ) {
# do stuff here.
}
substr( $_, 0, pos ) = "" if pos;
}
How do I substitute case insensitively on the LHS while preserving case on
the RHS?
Here's a lovely Perlish solution by Larry Rosler. It exploits properties
of bitwise xor on ASCII strings.
$_= "this is a TEsT case";
$old = 'test';
$new = 'success';
s{(\Q$old\E)}
{ uc $new | (uc $1 ^ $1) .
(uc(substr $1, -1) ^ substr $1, -1) x
(length($new) - length $1)
}egi;
print;
And here it is as a subroutine, modeled after the above:
sub preserve_case($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;
uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))
}
$a = "this is a TEsT case";
$a =~ s/(test)/preserve_case($1, "success")/egi;
print "$a\n";
This prints:
this is a SUcCESS case
As an alternative, to keep the case of the replacement word if it is longer
than the original, you can use this code, by Jeff Pinyan:
sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;
if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }
return uc $to | ($from ^ uc $from);
}
This changes the sentence to "this is a SUcCess case."
Just to show that C programmers can write C in any programming language, if
you prefer a more C-like solution, the following script makes the
substitution have the same case, letter by letter, as the original. (It
also happens to run about 240% slower than the Perlish solution runs.) If
the substitution has more characters than the string being substituted, the
case of the last character is used for the rest of the substitution.
# Original by Nathan Torkington, massaged by Jeffrey Friedl
#
sub preserve_case($$)
{
my ($old, $new) = @_;
my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
for ($i = 0; $i < $len; $i++) {
if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
$state = 0;
} elsif (lc $c eq $c) {
substr($new, $i, 1) = lc(substr($new, $i, 1));
$state = 1;
} else {
substr($new, $i, 1) = uc(substr($new, $i, 1));
$state = 2;
}
}
# finish up with any remaining new (for when new is longer than old)
if ($newlen > $oldlen) {
if ($state == 1) {
substr($new, $oldlen) = lc(substr($new, $oldlen));
} elsif ($state == 2) {
substr($new, $oldlen) = uc(substr($new, $oldlen));
}
}
return $new;
}
How can I make "\w" match national character sets?
Put "use locale;" in your script. The \w character class is taken from the
current locale.
See perllocale for details.
How can I match a locale-smart version of "/[a-zA-Z]/"?
You can use the POSIX character class syntax "/[[:alpha:]]/" documented in
perlre.
No matter which locale you are in, the alphabetic characters are the
characters in \w without the digits and the underscore. As a regex, that
looks like "/[^\W\d_]/". Its complement, the non-alphabetics, is then
everything in \W along with the digits and the underscore, or "/[\W\d_]/".
How can I quote a variable to use in a regex?
The Perl parser will expand $variable and @variable references in regular
expressions unless the delimiter is a single quote. Remember, too, that
the right-hand side of a "s///" substitution is considered a double-quoted
string (see perlop for more details). Remember also that any regex special
characters will be acted on unless you precede the substitution with \Q.
Here's an example:
$string = "Placido P. Octopus";
$regex = "P.";
$string =~ s/$regex/Polyp/;
# $string is now "Polypacido P. Octopus"
Because "." is special in regular expressions, and can match any single
character, the regex "P." here has matched the <Pl> in the original string.
To escape the special meaning of ".", we use "\Q":
$string = "Placido P. Octopus";
$regex = "P.";
$string =~ s/\Q$regex/Polyp/;
# $string is now "Placido Polyp Octopus"
The use of "\Q" causes the <.> in the regex to be treated as a regular
character, so that "P." matches a "P" followed by a dot.
What is "/o" really for?
Using a variable in a regular expression match forces a re-evaluation (and
perhaps recompilation) each time the regular expression is encountered.
The "/o" modifier locks in the regex the first time it's used. This always
happens in a constant regular expression, and in fact, the pattern was
compiled into the internal format at the same time your entire program was.
Use of "/o" is irrelevant unless variable interpolation is used in the
pattern, and if so, the regex engine will neither know nor care whether the
variables change after the pattern is evaluated the very first time.
"/o" is often used to gain an extra measure of efficiency by not performing
subsequent evaluations when you know it won't matter (because you know the
variables won't change), or more rarely, when you don't want the regex to
notice if they do.
For example, here's a "paragrep" program:
$/ = ''; # paragraph mode
$pat = shift;
while (<>) {
print if /$pat/o;
}
How do I use a regular expression to strip C style comments from a file?
While this actually can be done, it's much harder than you'd think. For
example, this one-liner
perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
will work in many but not all cases. You see, it's too simple-minded for
certain kinds of C programs, in particular, those with what appear to be
comments in quoted strings. For that, you'd need something like this,
created by Jeffrey Friedl and later modified by Fred Curtis.
$/ = undef;
$_ = <>;
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs
print;
This could, of course, be more legibly written with the "/x" modifier,
adding whitespace and comments. Here it is expanded, courtesy of Fred
Curtis.
s{
/\* ## Start of /* ... */ comment
[^*]*\*+ ## Non-* followed by 1-or-more *'s
(
[^/*][^*]*\*+
)* ## 0-or-more things which don't start with /
## but do end with '*'
/ ## End of /* ... */ comment
| ## OR various things which aren't comments:
(
" ## Start of " ... " string
(
\\. ## Escaped char
| ## OR
[^"\\] ## Non "\
)*
" ## End of " ... " string
| ## OR
' ## Start of ' ... ' string
(
\\. ## Escaped char
| ## OR
[^'\\] ## Non '\
)*
' ## End of ' ... ' string
| ## OR
. ## Anything other char
[^/"'\\]* ## Chars which doesn't start a comment, string or escape
)
}{$2}gxs;
A slight modification also removes C++ comments:
s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs;
Can I use Perl regular expressions to match balanced text?
Historically, Perl regular expressions were not capable of matching
balanced text. As of more recent versions of perl including 5.6.1
experimental features have been added that make it possible to do this.
Look at the documentation for the (??{ }) construct in recent perlre manual
pages to see an example of matching balanced parentheses. Be sure to take
special notice of the warnings present in the manual before making use of
this feature.
CPAN contains many modules that can be useful for matching text depending
on the context. Damian Conway provides some useful patterns in
Regexp::Common. The module Text::Balanced provides a general solution to
this problem.
One of the common applications of balanced text matching is working with
XML and HTML. There are many modules available that support these needs.
Two examples are HTML::Parser and XML::Parser. There are many others.
An elaborate subroutine (for 7-bit ASCII only) to pull out balanced and
possibly nested single chars, like "`" and "'", "{" and "}", or "(" and ")"
can be found in http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz
.
The C::Scan module from CPAN also contains such subs for internal use, but
they are undocumented.
What does it mean that regexes are greedy? How can I get around it?
Most people mean that greedy regexes match as much as they can.
Technically speaking, it's actually the quantifiers ("?", "*", "+", "{}")
that are greedy rather than the whole pattern; Perl prefers local greed and
immediate gratification to overall greed. To get non-greedy versions of
the same quantifiers, use ("??", "*?", "+?", "{}?").
An example:
$s1 = $s2 = "I am very very cold";
$s1 =~ s/ve.*y //; # I am cold
$s2 =~ s/ve.*?y //; # I am very cold
Notice how the second substitution stopped matching as soon as it
encountered "y ". The "*?" quantifier effectively tells the regular
expression engine to find a match as quickly as possible and pass control
on to whatever is next in line, like you would if you were playing hot
potato.
How do I process each word on each line?
Use the split function:
while (<>) {
foreach $word ( split ) {
# do something with $word here
}
}
Note that this isn't really a word in the English sense; it's just chunks
of consecutive non-whitespace characters.
To work with only alphanumeric sequences (including underscores), you might
consider
while (<>) {
foreach $word (m/(\w+)/g) {
# do something with $word here
}
}
How can I print out a word-frequency or line-frequency summary?
To do this, you have to parse out each word in the input stream. We'll
pretend that by word you mean chunk of alphabetics, hyphens, or
apostrophes, rather than the non-whitespace chunk idea of a word given in
the previous question:
while (<>) {
while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
$seen{$1}++;
}
}
while ( ($word, $count) = each %seen ) {
print "$count $word\n";
}
If you wanted to do the same thing for lines, you wouldn't need a regular
expression:
while (<>) {
$seen{$_}++;
}
while ( ($line, $count) = each %seen ) {
print "$count $line";
}
If you want these output in a sorted order, see perlfaq4: ``How do I sort a
hash (optionally by value instead of key)?''.
How can I do approximate matching?
See the module String::Approx available from CPAN.
How do I efficiently match many regular expressions at once?
The following is extremely inefficient:
# slow but obvious way
@popstates = qw(CO ON MI WI MN);
while (defined($line = <>)) {
for $state (@popstates) {
if ($line =~ /\b$state\b/i) {
print $line;
last;
}
}
}
That's because Perl has to recompile all those patterns for each of the
lines of the file. As of the 5.005 release, there's a much better
approach, one which makes use of the new "qr//" operator:
# use spiffy new qr// operator, with /i flag even
use 5.005;
@popstates = qw(CO ON MI WI MN);
@poppats = map { qr/\b$_\b/i } @popstates;
while (defined($line = <>)) {
for $patobj (@poppats) {
print $line if $line =~ /$patobj/;
}
}
Why don't word-boundary searches with "\b" work for me?
Two common misconceptions are that "\b" is a synonym for "\s+" and that
it's the edge between whitespace characters and non-whitespace characters.
Neither is correct. "\b" is the place between a "\w" character and a "\W"
character (that is, "\b" is the edge of a "word"). It's a zero-width
assertion, just like "^", "$", and all the other anchors, so it doesn't
consume any characters. perlre describes the behavior of all the regex
metacharacters.
Here are examples of the incorrect application of "\b", with fixes:
"two words" =~ /(\w+)\b(\w+)/; # WRONG
"two words" =~ /(\w+)\s+(\w+)/; # right
" =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
" =matchless= text" =~ /=(\w+)=/; # right
Although they may not do what you thought they did, "\b" and "\B" can still
be quite useful. For an example of the correct use of "\b", see the
example of matching duplicate words over multiple lines.
An example of using "\B" is the pattern "\Bis\B". This will find
occurrences of "is" on the insides of words only, as in "thistle", but not
"this" or "island".
Why does using $&, $`, or $' slow my program down?
Once Perl sees that you need one of these variables anywhere in the
program, it provides them on each and every pattern match. The same
mechanism that handles these provides for the use of $1, $2, etc., so you
pay the same price for each regex that contains capturing parentheses. If
you never use $&, etc., in your script, then regexes without capturing
parentheses won't be penalized. So avoid $&, $', and $` if you can, but if
you can't, once you've used them at all, use them at will because you've
already paid the price. Remember that some algorithms really appreciate
them. As of the 5.005 release. the $& variable is no longer "expensive"
the way the other two are.
What good is "\G" in a regular expression?
You use the "\G" anchor to start the next match on the same string where
the last match left off. The regular expression engine cannot skip over
any characters to find the next match with this anchor, so "\G" is similar
to the beginning of string anchor, "^". The "\G" anchor is typically used
with the "g" flag. It uses the value of pos() as the position to start the
next match. As the match operator makes successive matches, it updates
pos() with the position of the next character past the last match (or the
first character of the next match, depending on how you like to look at
it). Each string has its own pos() value.
Suppose you want to match all of consective pairs of digits in a string
like "1122a44" and stop matching when you encounter non-digits. You want
to match 11 and 22 but the letter <a> shows up between 22 and 44 and you
want to stop at "a". Simply matching pairs of digits skips over the "a" and
still matches 44.
$_ = "1122a44";
my @pairs = m/(\d\d)/g; # qw( 11 22 44 )
If you use the \G anchor, you force the match after 22 to start with the
"a". The regular expression cannot match there since it does not find a
digit, so the next match fails and the match operator returns the pairs it
already found.
$_ = "1122a44";
my @pairs = m/\G(\d\d)/g; # qw( 11 22 )
You can also use the "\G" anchor in scalar context. You still need the "g"
flag.
$_ = "1122a44";
while( m/\G(\d\d)/g )
{
print "Found $1\n";
}
After the match fails at the letter "a", perl resets pos() and the next
match on the same string starts at the beginning.
$_ = "1122a44";
while( m/\G(\d\d)/g )
{
print "Found $1\n";
}
print "Found $1 after while" if m/(\d\d)/g; # finds "11"
You can disable pos() resets on fail with the "c" flag. Subsequent matches
start where the last successful match ended (the value of pos()) even if a
match on the same string as failed in the meantime. In this case, the match
after the while() loop starts at the "a" (where the last match stopped),
and since it does not use any anchor it can skip over the "a" to find "44".
$_ = "1122a44";
while( m/\G(\d\d)/gc )
{
print "Found $1\n";
}
print "Found $1 after while" if m/(\d\d)/g; # finds "44"
Typically you use the "\G" anchor with the "c" flag when you want to try a
different match if one fails, such as in a tokenizer. Jeffrey Friedl offers
this example which works in 5.004 or later.
while (<>) {
chomp;
PARSER: {
m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; };
m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; };
m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; };
m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; };
}
}
For each line, the PARSER loop first tries to match a series of digits
followed by a word boundary. This match has to start at the place the last
match left off (or the beginning of the string on the first match). Since
"m/ \G( \d+\b )/gcx" uses the "c" flag, if the string does not match that
regular expression, perl does not reset pos() and the next match starts at
the same position to try a different pattern.
Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
While it's true that Perl's regular expressions resemble the DFAs
(deterministic finite automata) of the egrep(1) program, they are in fact
implemented as NFAs (non-deterministic finite automata) to allow
backtracking and backreferencing. And they aren't POSIX-style either,
because those guarantee worst-case behavior for all cases. (It seems that
some people prefer guarantees of consistency, even when what's guaranteed
is slowness.) See the book "Mastering Regular Expressions" (from O'Reilly)
by Jeffrey Friedl for all the details you could ever hope to know on these
matters (a full citation appears in perlfaq2).
What's wrong with using grep in a void context?
The problem is that grep builds a return list, regardless of the context.
This means you're making Perl go to the trouble of building a list that you
then just throw away. If the list is large, you waste both time and space.
If your intent is to iterate over the list, then use a for loop for this
purpose.
In perls older than 5.8.1, map suffers from this problem as well. But
since 5.8.1, this has been fixed, and map is context aware - in void
context, no lists are constructed.
How can I match strings with multibyte characters?
Starting from Perl 5.6 Perl has had some level of multibyte character
support. Perl 5.8 or later is recommended. Supported multibyte character
repertoires include Unicode, and legacy encodings through the Encode
module. See perluniintro, perlunicode, and Encode.
If you are stuck with older Perls, you can do Unicode with the
"Unicode::String" module, and character conversions using the
"Unicode::Map8" and "Unicode::Map" modules. If you are using Japanese
encodings, you might try using the jperl 5.005_03.
Finally, the following set of approaches was offered by Jeffrey Friedl,
whose article in issue #5 of The Perl Journal talks about this very matter.
Let's suppose you have some weird Martian encoding where pairs of ASCII
uppercase letters encode single Martian letters (i.e. the two bytes "CV"
make a single Martian letter, as do the two bytes "SG", "VS", "XX", etc.).
Other bytes represent single characters, just like ASCII.
So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the nine
characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
Now, say you want to search for the single character "/GX/". Perl doesn't
know about Martian, so it'll find the two bytes "GX" in the "I am CVSGXX!"
string, even though that character isn't there: it just looks like it is
because "SG" is next to "XX", but there's no real "GX". This is a big
problem.
Here are a few ways, all painful, to deal with it:
$martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian''
# bytes are no longer adjacent.
print "found GX!\n" if $martian =~ /GX/;
Or like this:
@chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
# above is conceptually similar to: @chars = $text =~ m/(.)/g;
#
foreach $char (@chars) {
print "found GX!\n", last if $char eq 'GX';
}
Or like this:
while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
print "found GX!\n", last if $1 eq 'GX';
}
Here's another, slightly less painful, way to do it from Benjamin Goldberg:
$martian =~ m/
(?!<[A-Z])
(?:[A-Z][A-Z])*?
GX
/x;
This succeeds if the "martian" character GX is in the string, and fails
otherwise. If you don't like using (?!<), you can replace (?!<[A-Z]) with
(?:^|[^A-Z]).
It does have the drawback of putting the wrong thing in $-[0] and $+[0],
but this usually can be worked around.
How do I match a pattern that is supplied by the user?
Well, if it's really a pattern, then just use
chomp($pattern = <STDIN>);
if ($line =~ /$pattern/) { }
Alternatively, since you have no guarantee that your user entered a valid
regular expression, trap the exception this way:
if (eval { $line =~ /$pattern/ }) { }
If all you really want to search for a string, not a pattern, then you
should either use the index() function, which is made for string searching,
or if you can't be disabused of using a pattern match on a non-pattern,
then be sure to use "\Q"..."\E", documented in perlre.
$pattern = <STDIN>;
open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
while (<FILE>) {
print if /\Q$pattern\E/;
}
close FILE;
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. All rights
reserved.
This documentation is free; you can redistribute it and/or modify it under
the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are hereby
placed into the public domain. You are permitted and encouraged to use
this code in your own programs for fun or for profit as you see fit. A
simple comment in the code giving credit would be courteous but is not
required.
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Index for
Section 1 |
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Alphabetical
listing for P |
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Top of
page |
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