CODEWITHSUNDEEP
perl
user1527651
user1527651

Reputation: 405

Difference between /Regex/gm and m/Regex/g in perl string matching

Is there a difference between ($ipAddrResult =~ /Regex/gm) and ($ipAddrResult =~ m/Regex/g) in perl string matching? When I google online I get explanation for second one and not the first one. The file I tried to edit has first condition.

Upvotes: 16

Views: 30473

Answers (3)

DavidM
DavidM

Reputation: 1437

In the expression

/Regex/gm

The "m" stands for multi-line matching. In the expression:

m/Regex/g

The "m" stands for "match" as opposed to a substitution, which looks like this:

s/Regex/replacement/g

Because matching (vs. substitution) is the default, you can generally leave off the "m/" from the start of the expression. In other words "m/Regex/g" is just a synonym for "/Regex/g".

Upvotes: 12

friedo
friedo

Reputation: 67028

The ms in different places mean different things.

Let's look at the second example first.

m// is the regular expression matching operator. As a shortcut, the m can be omitted, so 

$foo =~ m/$pattern/;

is exactly the same as

$foo =~ /$pattern/;

The only time the m is required is if you want to use delimiters other than / for your pattern. You can do, for example

$foo =~ m!$pattern!;

or

$foo =~ m[$pattern];

and so on, but these all require the m to be there.

In the first example, the m after the regex is a modifier flag which tells the regex how to behave. The regex flags are documented in the perlre man page, which has this to say:

m - Treat string as multiple lines. That is, change "^" and "$" from matching the start or end of line only at the left and right ends of the string to matching them anywhere within the string.

So this:

$foo =~ /$pattern/m;

is the same as this:

$foo =~ m/$pattern/m;

and the same as this:

$foo =~ m{$pattern}m;

Upvotes: 11

tripleee
tripleee

Reputation: 189679

Yes, m/regex/g is syntactically equivalent to just /regex/g. That is, it doesn't activate the /m flag at all. Compare to s/foo/bar/ which is not at all the same as s/foo/bar/s. The name m stands for "match" I believe.

Upvotes: 3

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