How do I use a variable as a regex modifier in Perl? - regex

I'm writing an abstraction function that will ask the user a given question and validate the answer based on a given regular expression. The question is repeated until the answer matches the validation regexp.
However, I also want the client to be able to specify whether the answer must match case-sensitively or not.
So something like this:
sub ask {
my ($prompt, $validationRe, $caseSensitive) = #_;
my $modifier = ($caseSensitive) ? "" : "i";
my $ans;
my $isValid;
do {
print $prompt;
$ans = <>;
chomp($ans);
# What I want to do that doesn't work:
# $isValid = $ans =~ /$validationRe/$modifier;
# What I have to do:
$isValid = ($caseSensitive) ?
($ans =~ /$validationRe/) :
($ans =~ /$validationRe/i);
} while (!$isValid);
return $ans;
}
Upshot: is there a way to dynamically specify a regular expression's modifiers?

Upshot: is there a way to dynamically specify a regular expression's modifiers?
From perldoc perlre:
"(?adlupimsx-imsx)"
"(?^alupimsx)"
One or more embedded pattern-match modifiers, to be turned on (or
turned off, if preceded by "-") for the remainder of the pattern or
the remainder of the enclosing pattern group (if any).
This is particularly useful for dynamic patterns, such as those read
in from a configuration file, taken from an argument, or specified in
a table somewhere. Consider the case where some patterns want to be
case-sensitive and some do not: The case-insensitive ones merely need
to include "(?i)" at the front of the pattern.
Which gives you something along the lines of
$isValid = $ans =~ m/(?$modifier)$validationRe/;
Just be sure to take the appropriate security precautions when accepting user input in this way.

You might also like the qr operator which quotes its STRING as a regular expression.
my $rex = qr/(?$mod)$pattern/;
$isValid = <STDIN> =~ $rex;

Get rid of your $caseSensitive parameter, as it will be useless in many cases. Instead, users of that function can encode the necessary information directly in the $validationRe regex.
When you create a regex object like qr/foo/, then the pattern is at that point compiled into instructions for the regex engine. If you stringify a regex object, you'll get a string that when interpolated back into a regex will have exactly the same behaviour as the original regex object. Most importantly, this means that all flags provided or omitted from the regex object literal will be preserved and can't be overridden! This is by design, so that a regex object will continue to behave identical no matter what context it is used in.
That's a bit dry, so let's use an example. Here is a match function that tries to apply a couple similar regexes to a list of strings. Which one will match?
use strict;
use warnings;
use feature 'say';
# This sub takes a string to match on, a regex, and a case insensitive marker.
# The regex will be recompiled to anchor at the start and end of the string.
sub match {
my ($str, $re, $i) = #_;
return $str =~ /\A$re\z/i if $i;
return $str =~ /\A$re\z/;
}
my #words = qw/foo FOO foO/;
my $real_regex = qr/foo/;
my $fake_regex = 'foo';
for my $re ($fake_regex, $real_regex) {
for my $i (0, 1) {
for my $word (#words) {
my $match = 0+ match($word, $re, $i);
my $output = qq("$word" =~ /$re/);
$output .= "i" if $i;
say "$output\t-->" . uc($match ? "match" : "fail");
}
}
}
Output:
"foo" =~ /foo/ -->MATCH
"FOO" =~ /foo/ -->FAIL
"foO" =~ /foo/ -->FAIL
"foo" =~ /foo/i -->MATCH
"FOO" =~ /foo/i -->MATCH
"foO" =~ /foo/i -->MATCH
"foo" =~ /(?^:foo)/ -->MATCH
"FOO" =~ /(?^:foo)/ -->FAIL
"foO" =~ /(?^:foo)/ -->FAIL
"foo" =~ /(?^:foo)/i -->MATCH
"FOO" =~ /(?^:foo)/i -->FAIL
"foO" =~ /(?^:foo)/i -->FAIL
First, we should notice that the string representation of regex objects has this weird (?^:...) form. In a non-capturing group (?: ... ), modifiers for the pattern inside the group can be added or removed between the question mark and colon, while the ^ indicates the default set of flags.
Now when we look at the fake regex that's actually just a string being interpolated, we can see that the addition of the /i flag makes a difference as expected. But when we use a real regex object, it doesn't change anything: The outside /i cannot override the (?^: ... ) flags.
It is probably best to assume that all regexes already are regex objects and should not be interfered with. If you load the regex patterns from a file, you should require the regexes to use the (?: ... ) syntax to apply flages (e.g. (?^i:foo) as an equivalent to qr/foo/i). E.g. loading one regex per line from a file handle could look like:
my #regexes;
while (<$fh>) {
chomp;
push #regexes, qr/$_/; # will die here on regex syntax errors
}

You need to use the eval function. The below code will work:
sub ask {
my ($prompt, $validationRe, $caseSensitive) = #_;
my $modifier = ($caseSensitive) ? "" : "i";
my $ans;
my $isValid;
do {
print $prompt;
$ans = <>;
chomp($ans);
# What I want to do that doesn't work:
# $isValid = $ans =~ /$validationRe/$modifier;
$isValid = eval "$ans =~ /$validationRe/$modifier";
# What I have to do:
#$isValid = ($caseSensitive) ?
# ($ans =~ /$validationRe/) :
# ($ans =~ /$validationRe/i);
} while (!$isValid);
return $ans;
}

Related

extract string between two dots

I have a string of the following format:
word1.word2.word3
What are the ways to extract word2 from that string in perl?
I tried the following expression but it assigns 1 to sub:
#perleval $vars{sub} = $vars{string} =~ /.(.*)./; 0#
EDIT:
I have tried several suggestions, but still get the value of 1. I suspect that the entire expression above has a problem in addition to parsing. However, when I do simple assignment, I get the correct result:
#perleval $vars{sub} = $vars{string} ; 0#
assigns word1.word2.word3 to variable sub
. has a special meaning in regular expressions, so it needs to be escaped.
.* could match more than intended. [^.]* is safer.
The match operator (//) simply returns true/false in scalar context.
You can use any of the following:
$vars{sub} = $vars{string} =~ /\.([^.]*)\./ ? $1 : undef;
$vars{sub} = ( $vars{string} =~ /\.([^.]*)\./ )[0];
( $vars{sub} ) = $vars{string} =~ /\.([^.]*)\./;
The first one allows you to provide a default if there's no match.
Try:
/\.([^\.]+)\./
. has a special meaning and would need to be escaped. Then you would want to capture the values between the dots, so use a negative character class like ([^\.]+) meaning at least one non-dot. if you use (.*) you will get:
word1.stuff1.stuff2.stuff3.word2 to result in:
stuff1.stuff2.stuff3
But maybe you want that?
Here is my little example, I do find the perl one liners a little harder to read at times so I break it out:
use strict;
use warnings;
if ("stuff1.stuff2.stuff3" =~ m/\.([^.]+)\./) {
my $value = $1;
print $value;
}
else {
print "no match";
}
result
stuff2
. has a special meaning: any character (see the expression between your parentheses)
Therefore you have to escape it (\.) if you search a literal dot:
/\.(.*)\./
You've got to make sure you're asking for a list when you do the search.
my $x= $string =~ /look for (pattern)/ ;
sets $x to 1
my ($x)= $string =~ /look for (pattern)/ ;
sets $x to pattern.

Why can't I store a regexp in a variable?

Given the following code,
my $string = "foo";
my $regex = s/foo/bar/;
$string =~ $regex;
print $string, "\n";
I would have expected the output to be bar, however it is foo. Why is that the case, and how can I solve that problem?
Note that in my actual case, the regex is more complicated, and I actually want to store several of them in a hash (so I can write something like $string =~ $rules{$key}).
You're looking for substitution, not only the regex part so I guess compiled regex (qr//) is not what you're looking for,
use strict;
use warnings;
my $string = "foo";
my $regex = sub { $_[0] =~ s/foo/bar/ };
$regex->($string);
print $string, "\n";
Your statement
my $regex = s/foo/bar/
is equivalent to
my $regex = $_ =~ s/foo/bar/
s/// returns the number of substitutions made, or it returns false (specifically, the empty string). So $regex is now '' or 1 (it could be more if the /g modifier was in effect) and
$string =~ $regex
is doing 'foo' =~ // or 'foo' =~ /1/ depending on what $_ contained originally.
You can store a regex pattern in a variable but, in your example, the regex is just foo, and there is a lot more going on than just that pattern
The statement s/foo/bar/ is more complex than it seems -- it is a fully-fledged statement that applies a regex pattern to a target string and substitutes a replacement string if the pattern is found. In this case the target string is the default variable $_ and the replacement string is foo. You could think of it as a call to a subroutine
substitute($_, 'foo', 'bar')
and the regex pattern is only the second parameter
What you can do is store a regex pattern. The regex part of that substitution is foo, and you can say
my $pattern = qr/foo/;
s/$pattern/bar/;
But you really should explain the problem that you're trying to solve so that we can help you better
In the assignment, you need to tell Perl not to evaluate the regular expression but just to keep it. This is what qr is for.
But you can't do this with whole substitutions, which is why Сухой27 suggests using a subroutine.

Perl how do you assign a varanble to a regex match result

How do you create a $scalar from the result of a regex match?
Is there any way that once the script has matched the regex that it can be assigned to a variable so it can be used later on, outside of the block.
IE. If $regex_result = blah blah then do something.
I understand that I should make the regex as non-greedy as possible.
#!/usr/bin/perl
use strict;
use warnings;
# use diagnostics;
use Win32::OLE;
use Win32::OLE::Const 'Microsoft Outlook';
my #Qmail;
my $regex = "^\\s\*owner \#";
my $sentence = $regex =~ "/^\\s\*owner \#/";
my $outlook = Win32::OLE->new('Outlook.Application')
or warn "Failed Opening Outlook.";
my $namespace = $outlook->GetNamespace("MAPI");
my $folder = $namespace->Folders("test")->Folders("Inbox");
my $items = $folder->Items;
foreach my $msg ( $items->in ) {
if ( $msg->{Subject} =~ m/^(.*test alert) / ) {
my $name = $1;
print " processing Email for $name \n";
push #Qmail, $msg->{Body};
}
}
for(#Qmail) {
next unless /$regex|^\s*description/i;
print; # prints what i want ie lines that start with owner and description
}
print $sentence; # prints ^\\s\*offense \ # not lines that start with owner.
One way is to verify a match occurred.
use strict;
use warnings;
my $str = "hello what world";
my $match = 'no match found';
my $what = 'no what found';
if ( $str =~ /hello (what) world/ )
{
$match = $&;
$what = $1;
}
print '$match = ', $match, "\n";
print '$what = ', $what, "\n";
Use Below Perl variables to meet your requirements -
$` = The string preceding whatever was matched by the last pattern match, not counting patterns matched in nested blocks that have been exited already.
$& = Contains the string matched by the last pattern match
$' = The string following whatever was matched by the last pattern match, not counting patterns matched in nested blockes that have been exited already. For example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
The match of a regex is stored in special variables (as well as some more readable variables if you specify the regex to do so and use the /p flag).
For the whole last match you're looking at the $MATCH (or $& for short) variable. This is covered in the manual page perlvar.
So say you wanted to store your last for loop's matches in an array called #matches, you could write the loop (and for some reason I think you meant it to be a foreach loop) as:
my #matches = ();
foreach (#Qmail) {
next unless /$regex|^\s*description/i;
push #matches_in_qmail $MATCH
print;
}
I think you have a problem in your code. I'm not sure of the original intention but looking at these lines:
my $regex = "^\\s\*owner \#";
my $sentence = $regex =~ "/^\s*owner #/";
I'll step through that as:
Assign $regexto the string ^\s*owner #.
Assign $sentence to value of running a match within $regex with the regular expression /^s*owner $/ (which won't match, if it did $sentence will be 1 but since it didn't it's false).
I think. I'm actually not exactly certain what that line will do or was meant to do.
I'm not quite sure what part of the match you want: the captures, or something else. I've written Regexp::Result which you can use to grab all the captures etc. on a successful match, and Regexp::Flow to grab multiple results (including success statuses). If you just want numbered captures, you can also use Data::Munge
You can do the following:
my $str ="hello world";
my ($hello, $world) = $str =~ /(hello)|(what)/;
say "[$_]" for($hello,$world);
As you see $hello contains "hello".
If you have older perl on your system like me, perl 5.18 or earlier, and you use $ $& $' like codequestor's answer above, it will slow down your program.
Instead, you can use your regex pattern with the modifier /p, and then check these 3 variables: ${^PREMATCH}, ${^MATCH}, and ${^POSTMATCH} for your matching results.

How to user Perl to find all instances of strings that match a regexp?

How to user Perl to find and print all strings that match a regexp?
The following only finds the first match.
$text="?Adsfsadfgaasdf.
?Bafadfdsaadsfadsf.
xcxvfdgfdg";
if($text =~ m/\\?([^\.]+\.)/) {
print "$1\n";
}
EDIT1: /g doesn't work
#!/usr/bin/env perl
$text="?Adsfsadfgaasdf.
?Bafadfdsaadsfadsf.
xcxvfdgfdg";
if($text =~ m/\\?([^\.]+\.)/g) {
print "$1\n";
}
$ ./test.pl
?Adsfsadfgaasdf.
The problem is that the /g modifier does not use capture groups for multiple matches. You need to either iterate over the matches in scalar context, or catch the returned list in list context. For example:
use v5.10; # required for say()
$text="?Adsfsadfgaasdf.
?Bafadfdsaadsfadsf.
xcxvfdgfdg";
while ($text =~ /\?([^.]+\.)/g) { # scalar context
say $1;
}
for ($text =~ /\?[^.]+\./g) { # list context
say; # match is held in $_
}
Note in the second case, I skipped the parens, because in list context the whole match is returned if there are no parens. You may add parens to select part of the string.
Your version, using if, uses scalar context, which saves the position of the most recent match, but does not continue. A way to see what happens is:
if($text =~ m/\?([^\.]+\.)/g) {
print "$1\n";
}
say "Rest of string: ", substr $text, pos;
pos gives the position of the most recent match.
In previous answer #TLP correctly wrote that matching should be in list context.
use Data::Dumper;
$text="?Adsfsadfgaasdf.
?Bafadfdsaa.
dsfadsf.
xcxvfdgfdg";
#arr = ($text =~ /\?([^\.]+\.)/g);
print Dumper(#arr);
Expected result:
$VAR1 = 'Adsfsadfgaasdf.';
$VAR2 = 'Bafadfdsaa.';
You seem to be missing the /g flag, which tells perl to repeat the match as many times as possible.

How do I tokenise a word given tokens that are subsumed incompletely in the word?

I understand how to use regex in Perl in the following way:
$str =~ s/expression/replacement/g;
I understand that if any part of the expression is enclosed in parentheses, it can be used and captured in the replacement part, like this:
$str =~ s/(a)/($1)dosomething/;
But is there a way to capture the ($1) above outside of the regex expression?
I have a full word which is a string of consonants, e.g. bEdmA, its vowelized version baEodamaA (where a and o are vowels), as well its split up form of two tokens, separated by space, bEd maA. I want to just pick up the vowelized form of the tokens from the full word, like so: beEoda, maA. I'm trying to capture the token within the full word expression, so I have:
$unvowelizedword = "bEdmA";
$tokens[0] = "bEd", $tokens[1] = "mA";
$vowelizedword = "baEodamA";
foreach $t(#tokens) {
#find the token within the full word, and capture its vowels
}
I'm trying to do something like this:
$vowelizedword = m/($t)/;
This is completely wrong for two reasons: the token $t is not present in exactly its own form, such as bEd, but something like m/b.E.d/ would be more relevant. Also, how do I capture it in a variable outside the regular expression?
The real question is: how can I capture the vowelized sequences baEoda and maA, given the tokens bEd, mA from the full word beEodamaA?
Edit
I realized from all the answers that I missed out two important details.
Vowels are optional. So if the tokens are : "Al" and "ywm", and the fully vowelized word is "Alyawmi", then the output tokens would be "Al" and "yawmi".
I only mentioned two vowels, but there are more, including symbols made up of two characters, like '~a'. The full list (although I don't think I need to mention it here) is:
#vowels = ('a', 'i', 'u', 'o', '~', '~a', '~i', '~u', 'N', 'F', 'K', '~N', '~K');
The following seems to do what you want:
#!/usr/bin/env perl
use warnings;
use strict;
my #tokens = ('bEd', 'mA');
my $vowelizedword = "beEodamaA";
my #regex = map { join('.?', split //) . '.?' } #tokens;
my $regex = join('|', #regex);
$regex = qr/($regex)/;
while (my ($matched) = $vowelizedword =~ $regex) {
$vowelizedword =~ s{$regex}{};
print "matched $matched\n";
}
Update as per your updated question (vowels are optional). It works from the end of the string so you'll have to gather the tokens into an array and print them in reverse:
#!/usr/bin/env perl
use warnings;
use strict;
my #tokens = ('bEd', 'mA', 'Al', 'ywm');
my $vowelizedword = "beEodamaA Alyawmi"; # Caveat: Without the space it won't work.
my #regex = map { join('.?', split //) . '.?$' } #tokens;
my $regex = join('|', #regex);
$regex = qr/($regex)/;
while (my ($matched) = $vowelizedword =~ $regex) {
$vowelizedword =~ s{$regex}{};
print "matched $matched\n";
}
Use the m// operator in so-called "list context", as this:
my #tokens = ($input =~ m/capturing_regex_here/modifiershere);
ETA: From what I understand now, what you were trying to say is that you want to match an optional vowel after each character of the tokens.
With this, you can tweak the $vowels variable to only contain the letters you seek. Optionally, you may also just use . to capture any character.
use strict;
use warnings;
use Data::Dumper;
my #tokens = ("bEd", "mA");
my $full = "baEodamA";
my $vowels = "[aeiouy]";
my #matches;
for my $rx (#tokens) {
$rx =~ s/.\K/$vowels?/g;
if ($full =~ /$rx/) {
push #matches, $full =~ /$rx/g;
}
}
print Dumper \#matches;
Output:
$VAR1 = [
'baEoda',
'mA'
];
Note that
... $full =~ /$rx/g;
does not require capturing groups in the regex.
I suspect that there is an easier way to do whatever you're trying to accomplish. The trick is not to make the regex generation code so tricky that you forget what it's actually doing.
I can only begin to guess at your task, but from your single example, it looks like you want to check that the two subtokens are in the larger token, ignoring certain characters. I'm going to guess that those sub tokens have to be in order and can't have anything else between them besides those vowel characters.
To match the tokens, I can use the \G anchor with the /g global flag in scalar context. This anchors the match to the character one after the end of the last match for the same scalar. This way allows me to have separate patterns for each sub token. This is much easier to manage since I only need to change the list of values in #subtokens.
Once you go through each of the pairs and find which ones match all the patterns, I can extract the original string from the pair.
use v5.14;
my $vowels = '[ao]*';
my #subtokens = qw(bEd mA);
# prepare the subtoken regular expressions
my #patterns = map {
my $s = join "$vowels", map quotemeta, (split( // ), '');
qr/$s/;
} #subtokens;
my #tokens = qw( baEodamA mAabaEod baEoda mAbaEoda );
my #grand_matches;
TOKEN: foreach my $token ( #tokens ) {
say "-------\nMatching $token..........";
my #matches;
PATTERN: foreach my $pattern ( #patterns ) {
say "Position is ", pos($token) // 0;
# scalar context /g and \G
next TOKEN unless $token =~ /\G($pattern)/g;
push #matches, $1;
say "Matched with $pattern";
}
push #grand_matches, [ $token, \#matches ];
}
# Now report the original
foreach my $tuple ( #grand_matches ) {
say "$tuple->[0] has both fragments: #{$tuple->[1]}";
}
Now, here's the nice thing about this structure. I've probably guessed wrong about your task. If I have, it's easy to fix without changing the setup. Let's say that the subtokens don't have to be in order. That's an easy change to the pattern I created. I just get rid of the
\G anchor and the /g flag;
next TOKEN unless $token =~ /($pattern)/;
Or, suppose that the tokens have to be in order, but other things may be between them. I can insert a .*? to match that stuff, effectively skipping over it:
next TOKEN unless $token =~ /\G.*?($pattern)/g;
It would be much nicer if I could manage all of this from the map where I create the patterns, but the /g flag isn't a pattern flag. It has to go with the operator.
I find it much easier to manage changing requirements when I don't wrap everything in a single regular expression.
Assuming the tokens need to appear in order and without anything (other than a vowel) between them:
my #tokens = ( "bEd", "mA" );
my $vowelizedword = "baEodamaA";
my $vowels = '[ao]';
my (#vowelized_sequences) = $vowelizedword =~ ( '^' . join( '', map "(" . join( $vowels, split( //, $_ ) ) . "(?:$vowels)?)", #tokens ) . '\\z' );
print for #vowelized_sequences;