Matching Dollar Sign in Perl String

Question

A simple text string which contains a dollar ($) sign in Perl program:

open my $fh, "<", $fp or die "can't read open '$fp': $OS_ERROR";
  while (<$fh>)
  {
    $line=''; #Initialize the line variable
    $line=$_; #Reading a record from a text file
    print "Line is $line\n"; #Printing for confirming
    (@arr)=split('\|',$line);
    

$line gets the following pipe-separated string (confirmed by printing $line value):

Vanilla Cake $3.65 New Offering|Half pound Vanilla Cake||Cake with vanilla, cream and cheese

then split and pull that record into specific array elements:

(@arr)=split('\|',$line);

$arr[0] gets Vanilla Cake $3.65, $arr1 gets Half pound Vanilla Cake, $arr[2] remains empty/NULL, $arr[3] gets Cake with vanilla, cream and cheese

Now I check if $arr[0] contains a price value. Pattern to match is some text (Vanilla Cake ), then a dollar sign($), followed by one or more digits (value of 3 in this case), decimal is optional - may be there or may not be there, and then there can be one or more digits after decimal (.65 in this case). Using the following regex:

if ($arr[0]=~ /(.*?)(\$\d+(?:\.\d+)?)/)
{
     print "match1 is $1, match2 is $2, match3 is $3, match4 is $4\n";
}

The problem is that $1, $2, $3, $4 - all matching pattern values are printing as NULL/EMPTY. I suppose it is because of the $ sign being a part of the string $arr[0].

My guess is that because of $3.65 value, it is taking $3 part (before the decimal) as a variable and trying to substitute it and $3 is NULL. So the regex matching is happening buy value extraction may be failing because the whole string may be getting interpreted as Vanilla Cake .65, and not as Vanilla Cake $3.65 (This is my guess)

Probably, that's why the regex matching & extraction is failing.

I also read somewhere that it may be dependent on the variable initialization ($line or $arr[0] as single quote or double quote) - I have no clue about such a dependency (that's why included all the code like initialization of $line variable as above). $line reads one record from a file at a time, so needs to be initialized at each iteration.

Have tried solutions given in Escape a dollar sign inside a variable and Trouble escaping dollar sign in Perl, but unable to get it working. Other trial and errors on creating the regex on https://regex101.com/r/FQjcHp/2/ are also not helping.

Can someone please let me know how to get the values of Vanilla Cake and $3.65 from the above string using the right regex code?

PS: Adding a screenshot of online compiler run with same code, which works fine and captures $ value correctly. Somehow, in my program it is not picking it up.

Answer 1

I ran into a similar problem around 2 years back - and had to break my head for more than 5 days before I could get to the root of the issue with the great $ sign. Here's how it went:

Dollar regex value was not printing - something similar to what you are observing.

The perl code written ages ago by someone had initialized the string var with double quotes. Something like

$string="This is some text";

And it worked perfectly till I touched it. :-)

What I did was inserted a variable into it, like

$string="This is some $PriceVariableHavingDollarSign text";

and then I tried to run a dollar matching regex on the $string variable with a hope to detect the dollar. Not exactly, but something very similar to what you are trying to do as follows:

$string=~ /(.*?)(\$\d+(?:\.\d+)?)/

And it either gave compilation error, or failed to pickup the dollar sign completely with the different regex combinations I tried.

So my answer-cum-suggestion is to check in your "lengthy code" if something similar is happening with double quotes on your variable. Most probably, that may be causing the problem.

Before taking in the value at the source, if possible try to use \ on the $ sign, like (at least that solved my problem). Instead of

PriceVariableHavingDollarSign = "Cake is $3.5";

try having

$PriceVariableHavingDollarSign ="Cake is \$3.5";

Here is a great explanation of what happens with double quotes and single quotes in Perl. https://www.effectiveperlprogramming.com/2012/01/understand-the-order-of-operations-in-double-quoted-contexts/

And good job for the explicit details you've put in the question, comments and graphic. It helps you to get all possible angles, scenarios as well as solutions.