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Re: difference in regex

by haukex (Chancellor)
on May 29, 2018 at 14:03 UTC ( #1215373=note: print w/replies, xml ) Need Help??


in reply to difference in regex

You will find the answer to your question in "Regexp Quote-Like Operators" in perlop - basically, different regex operations have different return values in different contexts. See also perlretut for a tutorial.

Operation Context () Capturing
Groups
Return Value on Match
(and notes on behavior)
Return Value on Failure Example
m// scalar - true false
my $x = "foobar"=~/[aeiou]/; # => $x is true my $y = "foobar"=~/[xyz]/; # => $y is false
m//g scalar - true
(each execution of m//g finds the next match,
see "Global matching" in perlretut)
false if there is no further match
my $str = "foobar"; my $x = $str=~/[aeiou]/g; # matches first "o" => $x is true, pos($str) is 2 $x = $str=~/[aeiou]/g; # matches second "o" => $x is true, pos($str) is 3 $x = $str=~/[aeiou]/g; # matches "a" => $x is true, pos($str) is 5 $x = $str=~/[aeiou]/g; # no more matches => $x is false, pos($str) is undef
m// list no the list (1) the empty list ()
my ($x) = "foobar"=~/[aeiou]/; # => $x is 1
m//g list no a list of all the matched strings, as if there were parentheses around the whole pattern the empty list ()
my ($x,$y,$z) = "foobar"=~/[aeiou]/g; # => $x is "o", $y is "o", $z is "a"
m// list yes a list consisting of the subexpressions matched by the parentheses in the pattern, that is, ($1, $2, $3...) the empty list ()
my ($x,$y) = "foobar"=~/([aeiou])(.)/; # => $x is "o", $y is "o"
m//g list yes a list of the substrings matched by any capturing parentheses in the regular expression, that is, ($1, $2...) repeated for each match the empty list ()
my ($w,$x,$y,$z) = "foobar"=~/([aeiou])(.)/g; # => $w is "o", $x is "o", $y is "a", $z is "r"
s/// - - the number of substitutions made false
my $x = "foobar"; my $y = $x=~s/[aeiou]/x/g; # => $y is 3
s///r - - a copy of the original string with substitution(s) applied
(available since Perl 5.14)
the original string
my $x = "foobar"=~s/[aeiou]/x/gr; # => $x is "fxxbxr"

In this table, "true" and "false" refer to Perl's notion of Truth and Falsehood. Remember not to rely on any of the capture variables like $1, $2, etc. unless the match succeeds!

In my $foo = "bar"=~/a/;, the right-hand side of the assignment ("bar"=~/a/) is in scalar context. In my ($foo) = "bar"=~/a/; or my @foo = "bar"=~/a/;, the right-hand side is in list context. That's why, in your example, you need those parens in ($value): because you want the matching operation to return the contents of the capture group.

Note that your expressions can be slightly simplified, not all the parens you showed are needed:

my ($value) = $row =~ /.*,(.*)/; # and $row =~ s/,[^,]*$//;

A few additional comments on your code:

  • ($row =~ s/,[^,]*$//); # gets substring before the last comma - this comment isn't quite right or at least potentially misleading, since it deletes the string before after and including the last comma.
  • /.*,(.*)/ matches any comma anywhere in the string, for simple input strings it may behave correctly, but I'd strongly recommend coding more defensively and writing it like your second expression: my ($value) = $row=~/,([^,]*)$/; - the $ anchor makes sure that the regex only matches the last comma and what follows it (unless you use the /m modifier, since it changes the meaning of $).
  • While the use of Scalar::Util's looks_like_number is often a good idea, note that if you don't mind being a little more restrictive, Regexp::Common (or a hand-written regex) would allow you to combine the two regular expressions:
    use Regexp::Common qw/number/; my $row = "a,b,c,d,15"; if ( $row=~s/,($RE{num}{real})$// ) { print "matched <$1>\n"; } print "row is now <$row>\n"; __END__ matched <15> row is now <a,b,c,d>
  • If this is a CSV file, consider using Text::CSV (also install Text::CSV_XS for speed)

Update: Added s///r to the table and added a few more doc links. A few other edits and updates. 2019-02-16: Added "Return Value on Failure" column to table, and a few other small updates. 2019-08-17: Updated the link to "Truth and Falsehood".

Replies are listed 'Best First'.
Re^2: difference in regex
by ovedpo15 (Monk) on May 29, 2018 at 14:11 UTC
    Thank you for the replay!
    As I mentioned on one of the posts on this thread - I would like to split it somehow into two scalars. I can use my ($a,$b) = ($row=~ /(.*),(.*)/); But if $row doesn't have commas it won't work. how do I make always put a string into $path
    for example:
    if "abc" it will be $path = "abc" and $value is undefined.
    if "abc,5" it will be $path = "abc" and $value = 5
    if "a,b,c,5" it will be $path = "a,b,c" and $value = 5

      Although personally I'd still use a conditional, of course it's possible to do it all in one regex. One way is by making the comma optional by putting a ? on a group, in this case I'm using a non-capturing (?:...) group, and I had to make the first part of the regex non-greedy so that it doesn't swallow an existing comma:

      use warnings; use strict; use Test::More; my $regex = qr/ ^ (.*?) (?: , ([^,]*) )? $ /x; ok "abc"=~$regex; is $1, "abc"; is $2, undef; ok "abc,5"=~$regex; is $1, "abc"; is $2, 5; ok "a,b,c,5"=~$regex; is $1, "a,b,c"; is $2, 5; done_testing;

      Update: An alternative that says a little more explicitly: either match a string with no commas in it, or, if there are commas, I want to match the thing after the last one: /^ (?| ([^,]*) | (.*) , ([^,]*) ) $/x Update 2: And it turns out this regex is much faster than the above! (try using it in this benchmark)

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