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Diffstat (limited to 'pod/perlop.pod')
| -rw-r--r-- | pod/perlop.pod | 21 |
1 files changed, 17 insertions, 4 deletions
diff --git a/pod/perlop.pod b/pod/perlop.pod index 35b2fe4d7b..07c01a5279 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -2055,11 +2055,24 @@ The bottom line is that using C</o> is almost never a good idea. =item The empty pattern C<//> If the I<PATTERN> evaluates to the empty string, the last -I<successfully> matched regular expression is used instead. In this -case, only the C<g> and C<c> flags on the empty pattern are honored; -the other flags are taken from the original pattern. If no match has +I<successfully> matched regular expression is used instead. In this +case, only the C<g> and C<c> flags on the empty pattern are honored; the +other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine -empty pattern (which will always match). +empty pattern (which will always match). Using a user supplied string as +a pattern has the risk that if the string is empty that it triggers the +"last successful match" behavior, which can be very confusing. In such +cases you are recommended to replace C<m/$pattern/> with +C<m/(?:$pattern)/> to avoid this behavior. + +The last successful pattern may be accessed as a variable via +C<${^LAST_SUCCESSFUL_PATTERN}>. Matching against it, or the empty +pattern should have the same effect, with the exception that when there +is no last successful pattern the empty pattern will silently match, +whereas using the C<${^LAST_SUCCESSFUL_PATTERN}> variable will produce +undefined warnings (if warnings are enabled). You can check +C<defined(${^LAST_SUCCESSFUL_PATTERN})> to test if there is a "last +successful match" in the current scope. Note that it's possible to confuse Perl into thinking C<//> (the empty regex) is really C<//> (the defined-or operator). Perl is usually pretty |