Please correct me in the above regular expression

[suvendra123]

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[hippo]

See How to ask better questions using Test::More and sample data

use strict;
use warnings;

use Test::More tests => 1;

my $in = "int abc;\n.abc(abc)";
my $want = ['abc', '.abc', '(abc'];

my @have = $in =~ /([\.(]?abc)/g;

is_deeply \@have, $want;

[afoken]

Same monk, same problem as Unable to find my mistake and all six other threads started by suvendra123 so far.

Regular expression to match

int abc;

.abc(abc)

above as input and below as output is

.abc

(abc

abc

is

[\.(]abc

Please correct me

Please do not open new threads for old problems.

Show at least a little bit of respect for people trying to help you by reading and understanding the answers you were given before.

Perlmonks is not a code writing service, you are expected to learn from the answers you were given.

Alexander

[AnomalousMonk]

hippo has shown++ the regex you must use to match any or all of the substrings .abc (abc abc in the string "int abc;\n.abc(abc)".

A narrative explanation may be useful. The regex pattern [\.(] (or [.(] which I will use from here on; the escape is not needed in a character class, but does no harm) defines the characters in a class. The regex [.(]abc requires that a character from the class and the following 'abc' characters all be present for a match to occur.

    ++++------- a single character from this class (an "atom")
    ||||        is required
    vvvv
    [.(]abc
        ^^^
        |||
        +++---- all these characters are required

So 'int abc;' cannot match because 'abc' is not preceded by a required "atom": a character from the character class.

On the other hand, the regex pattern [.(]?abc can match something in 'int abc;' because the character class has been made optional by the ? (0 or 1) quantifier:

    ++++------- a character from this class is an "atom"
    ||||
    |||| +++--- all these characters are required
    |||| |||
    vvvv vvv
    [.(]?abc
        ^
        |
        +------ 0 or 1 of preceding "atom" is required

In the context of this discussion, an "atom" can be thought of as anything that can be quantified by the * ? + {} quantifiers; see discussions of quantifiers in Regular Expressions in perlre and Matching repetitions in perlretut.