Hi, I have a question on the capturing parentheses. I have made the following test sample to explore the necessity of the parentheses:

$str = "Ab stuff Cd stuff Ef stuff";

# case 1
@ary1 = $str =~ m/(stuff)/g ;

# case 2
@ary2 = $str =~ m/(?:stuff)/g ;

# case 3
@ary3 = $str =~ m/stuff/g ;

print "\@ary1 = @ary1\n";
print "\@ary2 = @ary2\n";
print "\@ary3 = @ary3\n";
[download]

All three cases return the same result. Explanation for case 1 is covered in earlier posts. However I am puzzled by the use of parentheses in the example, so I added ?: to it to tell the regular expression to forget the value in the capture parentheses if any. The result is the same! So the regular expression is not acting on the $1 variable captured by the parentheses at all. So I eliminated the parentheses totally, I still get the same result.

Ok, my instinct tells me that this Perl idiom is acting on the behaviour of m//g, or more specific the g modifier. It seems the g modifier introduces it's own pattern matching memory behaviour and discards the regular expression memory in some cases.

I looked up the perldoc, which states:
The /g modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern.

Ok, my question is, what is the expected behaviour of the g modifier? Why is the /g modifier capturing the value that I want it to forget (with ?:)? Is it a feature or bug? Or perhaps /(?:pattern)/ is equivalent to /pattern/?

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.