G'day Thai Heng,

Perhaps your wording, "alphabetic characters plus a space", is a clue to what you're not understanding. I probably would have said "alphabetic characters or a space".

Some documentation that may help you:

There are also some tools that can help you to understand regex patterns. I see you've already been shown "use re 'debug';": you may have found the output somewhat esoteric. Here's a couple more that may be more suitable.

Damian Conway's Regexp::Debugger is a particular favourite of mine. It provides a dynamic explanation of each step that the regex engine performs. It's output shows: the position in the string and the part of the regex trying to match at this position; what's been matched so far; what values $1, $2, etc. currently hold; and so on. The simplest usage is to just add use Regexp::Debugger; near the top of your code; run your code; and use 's' to step through each operation. See the documentation for more commands and other information.

YAPE::Regex::Explain is easy to use. It provides a static explanation of the regex you give it. While you're learning, and using regexes such as you have here, this should be fine; when you start looking at newer, more advanced regex constructs, this module won't help you (see its LIMITATIONS section for more details). 

#!/usr/bin/env perl

use strict;
use warnings;

use YAPE::Regex::Explain;

my $re = qr{Name:\s+([[:alpha:] ]+?)\s+Age:\s+(\d+)};

print YAPE::Regex::Explain::->new($re)->explain;

[download]

Output:

The regular expression:

(?-imsx:Name:\s+([[:alpha:] ]+?)\s+Age:\s+(\d+))

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):
----------------------------------------------------------------------
  Name:                    'Name:'
----------------------------------------------------------------------
  \s+                      whitespace (\n, \r, \t, \f, and " ") (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [[:alpha:] ]+?           any character of: letters, ' ' (1 or
                             more times (matching the least amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \s+                      whitespace (\n, \r, \t, \f, and " ") (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  Age:                     'Age:'
----------------------------------------------------------------------
  \s+                      whitespace (\n, \r, \t, \f, and " ") (1 or
                           more times (matching the most amount
                           possible))
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

-- Ken

In reply to Re: a regex pattern how to understand by kcott
in thread a regex pattern how to understand by Thai Heng

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