如果子模式捕获不匹配，是否有 Perl 正则表达式元字符或指定默认值的方法？

Question

Here's the idea. I am parsing command line options but doing it across the entire command line, not by each @ARGV element separately.

program --format="%H:%M:%S" --timeout 12 --nofail

I want the parsing to work with these cases.

1. --name=value , easy to parse
2. --name value, pretty easy
3. --name no value , default the value to 1

Here is the regex which works, except it cannot do the missing value case

%options = "@ARGV" ~= /--([A-Za-z]+)[= ]([^-]\S*)/g;

ie match --name=value or --name value but not --name --name , --name --name is two names, not a --name=value pair.

If a --name has no value following it that matches the second capture in the regex, is there a way, within the regex, to specify a default, in my case a 1, to indicate "true". ie if an --name has no argument, like --nofail then set that argument to 1 indicating true.

Actually, in asking this I figured out a workaround using separate match statements which is fine. However, just out of curiosity, the question still stands, is there a Perl regex way to have a default if a submatch fails?

Answer 1

I don't see how to return a list reflecting a changed input. To change the input we need s{}{}xr operator, so we need code in its replacement part to analyze captures and decide what to change; and, we get a string, not a list, which need be further processed (split).

Here is then one such take, with a minimal intrusion of code.

Match name and value , with = or space between them, and if value ( $2 ) is undefined give it a value; so we need /x to do that " if... ". Once we are at it, put a space between all name-value pairs. This goes under /r so that the changed string is returned, and passed through split

my %arg = split ' ', 
    $args =~ s{ --(\w+) (?: =|\s+|\z) ([^-]\S*)? }{ $1 . " " . ($2//"10 ") }ergx; 

The split can be done by another regex instead but that's still extra processing.

A complete program (where I added more flags to input)

use warnings;
use strict;
use feature 'say';

my $args = shift // q(--fmt="%H:%M" --f1 --time 12 --f2 --f3);      
say $args;

my %arg = split ' ',
   $args =~ s{ --(\w+) (?: =|\s+|\z) ([^-]\S*)? }{ $1 . ' ' . ($2//'1 ') }ergx; 

say "$_ => $arg{$_}" for keys %arg;

This prints as expected.

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I am not sure why to use regex here. There are libraries for this, like Getopt::Long . If this is somehow not possible then processing @ARGV term by term goes nice and easy -- and is fast .