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Diffstat (limited to 'pod/perlop.pod')
| -rw-r--r-- | pod/perlop.pod | 62 |
1 files changed, 44 insertions, 18 deletions
diff --git a/pod/perlop.pod b/pod/perlop.pod index 574e9238d8..9e1e3f14d0 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -414,7 +414,7 @@ can assign to them): Note that this is not guaranteed to contribute to the readability of your program. -=head2 Assigment Operators +=head2 Assignment Operators "=" is the ordinary assignment operator. @@ -463,8 +463,9 @@ argument and returns that value. This is just like C's comma operator. In a list context, it's just the list argument separator, and inserts both its arguments into the list. -The => digraph is simply a synonym for the comma operator. It's useful -for documenting arguments that come in pairs. +The => digraph is mostly just a synonym for the comma operator. It's useful for +documenting arguments that come in pairs. As of 5.001, it also forces +any word to the left of it to be interpreted as a string. =head2 List Operators (Rightward) @@ -622,8 +623,8 @@ interpolating won't change over the life of the script. However, mentioning C</o> constitutes a promise that you won't change the variables in the pattern. If you change them, Perl won't even notice. -If the PATTERN evaluates to a null string, the most recently executed -(and successfully compiled) regular expression is used instead. +If the PATTERN evaluates to a null string, the last +successfully executed regular expression is used instead. If used in a context that requires a list value, a pattern match returns a list consisting of the subexpressions matched by the parentheses in the @@ -745,7 +746,7 @@ PATTERN contains a $ that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you only want the pattern compiled once the first time the variable is interpolated, use the C</o> option. If the pattern -evaluates to a null string, the most recently executed (and successfully compiled) regular +evaluates to a null string, the last successfully executed regular expression is used instead. See L<perlre> for further explanation on these. Options are: @@ -797,9 +798,9 @@ Examples: # Delete C comments. $program =~ s { - /\* (?# Match the opening delimiter.) - .*? (?# Match a minimal number of characters.) - \*/ (?# Match the closing delimiter.) + /\* # Match the opening delimiter. + .*? # Match a minimal number of characters. + \*/ # Match the closing delimiter. } []gsx; s/^\s*(.*?)\s*$/$1/; # trim white space @@ -997,15 +998,16 @@ If the string inside the angle brackets is a reference to a scalar variable (e.g. <$foo>), then that variable contains the name of the filehandle to input from. -If the string inside angle brackets is not a filehandle, it is -interpreted as a filename pattern to be globbed, and either a list of -filenames or the next filename in the list is returned, depending on -context. One level of $ interpretation is done first, but you can't -say C<E<lt>$fooE<gt>> because that's an indirect filehandle as explained in the -previous paragraph. You could insert curly brackets to force -interpretation as a filename glob: C<E<lt>${foo}E<gt>>. (Alternately, you can -call the internal function directly as C<glob($foo)>, which is probably -the right way to have done it in the first place.) Example: +If the string inside angle brackets is not a filehandle, it is interpreted +as a filename pattern to be globbed, and either a list of filenames or the +next filename in the list is returned, depending on context. One level of +$ interpretation is done first, but you can't say C<E<lt>$fooE<gt>> +because that's an indirect filehandle as explained in the previous +paragraph. In older version of Perl, programmers would insert curly +brackets to force interpretation as a filename glob: C<E<lt>${foo}E<gt>>. +These days, it's consdired cleaner to call the internal function directly +as C<glob($foo)>, which is probably the right way to have done it in the +first place.) Example: while (<*.c>) { chmod 0644, $_; @@ -1030,6 +1032,30 @@ and just do your own grep() on the filenames. Furthermore, due to its current implementation of using a shell, the glob() routine may get "Arg list too long" errors (unless you've installed tcsh(1L) as F</bin/csh>). +A glob only evaluates its (embedded) argument when it is starting a new +list. All values must be read before it will start over. In a list +context this isn't important, because you automatically get them all +anyway. In a scalar context, however, the operator returns the next value +each time it is called, or a FALSE value if you've just run out. Again, +FALSE is returned only once. So if you're expecting a single value from +a glob, it is much better to say + + ($file) = <blurch*>; + +than + + $file = <blurch*>; + +because the latter will alternate between returning a filename and +returning FALSE. + +It you're trying to do variable interpolation, it's definitely better +to use the glob() function, because the older notation can cause people +to become confused with the indirect filehandle notatin. + + @files = glob("$dir/*.[ch]"); + @files = glob($files[$i]); + =head2 Constant Folding Like C, Perl does a certain amount of expression evaluation at |