mysterious regexp syntax

grizzley has asked for the wisdom of the Perl Monks concerning the following question:

I have following entry in some script

# Find LOGIN tag.
        if ((($ln =~ ?<[     ]*LOGIN[     ]?) || 
             ($ln =~ ?<[     ]*LOGIN$?)) && 
             ($pword) && ($uname)) {
...
}
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It is working, but how? I'm sure match should start with 'm' letter if delimiter is not '/', but this '?' clearly is not delimiter. It is not assertion as well because assertion starts with '?<='. Looks like '<' is just part of pattern searching for '<LOGIN' (with optional white characters). '?' is special character and perl should give me some syntax error like:

C:\>perl -e "/?/"
Quantifier follows nothing in regex; marked by <-- HERE in m/? <-- HER
+E / at -e line 1.
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and yet it just works. Deparse does not give any clue:

# perl -MO=Deparse -e '$ln=" < LOGIN > "; $ln =~ ?<[ ]*LOGIN[ ]?'
$ln = ' < LOGIN > ';
$ln =~ ?<[ ]*LOGIN[ ]?;
-e syntax OK
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neither does Regex::Explain:

C:\>perl -e "use YAPE::Regex::Explain; $REx = '?<[ ]*LOGIN[ ]?'; print
+ $exp = YAPE::Regex::Explain->new($REx)->explain"
The regular expression:



matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

C:\>perl -e "use YAPE::Regex::Explain; $REx = qr/?<[ ]*LOGIN[ ]?/; pri
+nt $exp = YAPE::Regex::Explain->new($REx)->explain"
Quantifier follows nothing in regex; marked by <-- HERE in m/? <-- HER
+E <[ ]*LOGIN[ ]?/ at -e line 1.
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Perl version is 5.8.8 on Linux and 5.14.2 on Windows.

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Re: mysterious regexp syntax
by Tux (Canon) on Feb 04, 2014 at 08:46 UTC

$ man perlop
:
:
               If "/" is the delimiter then the initial "m" is optiona
+l.  With
               the "m" you can use any pair of non-whitespace (ASCII)
               characters as delimiters.  This is particularly useful 
+for
               matching path names that contain "/", to avoid LTS (lea
+ning
               toothpick syndrome).  If "?" is the delimiter, then a m
+atch-
               only-once rule applies, described in "m?PATTERN?" below
+.  If
               "'" is the delimiter, no interpolation is performed on 
+the
               PATTERN.  When using a character valid in an identifier
+,
               whitespace is required after the "m".
:
:
       m?PATTERN?msixpodualgc
       ?PATTERN?msixpodualgc
               This is just like the "m/PATTERN/" search, except that 
+it
               matches only once between calls to the reset() operator
+.  This
               is a useful optimization when you want to see only the 
+first
               occurrence of something in each file of a set of files,
+ for
               instance.  Only "m??"  patterns local to the current pa
+ckage
               are reset.
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Enjoy, Have FUN! H.Merijn

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Re^2: mysterious regexp syntax

by grizzley (Chaplain) on Feb 04, 2014 at 10:29 UTC

# perl -le '$_="abc"; print 1 if ?b?'
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Re^3: mysterious regexp syntax

by Athanasius (Archbishop) on Feb 04, 2014 at 12:51 UTC

But note the following, from perl5140delta:

?PATTERN? (without the initial m) has been deprecated and now produces a warning. This is to allow future use of ? in new operators. The match-once functionality is still available as m?PATTERN?.

So with v5.14.4, I get:

22:39 >perl -le "$_ = qq[abc]; print 1 if ?b?;"
Use of ?PATTERN? without explicit operator is deprecated at -e line 1.
1

22:39 >
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Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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