If you're dealing only with regex operators supported by Perl version 5.6 and before (as you are in the OPed example), the YAPE::Regex::Explain module can sometimes be helpful:
c:\@Work\Perl\monks>perl -wMstrict -le
"use YAPE::Regex::Explain;
;;
print YAPE::Regex::Explain->new('^(.+)(e|r)(.*)$')->explain;
"
The regular expression:
(?-imsx:^(.+)(e|r)(.*)$)
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):
----------------------------------------------------------------------
^ the beginning of the string
----------------------------------------------------------------------
( group and capture to \1:
----------------------------------------------------------------------
.+ any character except \n (1 or more times
(matching the most amount possible))
----------------------------------------------------------------------
) end of \1
----------------------------------------------------------------------
( group and capture to \2:
----------------------------------------------------------------------
e 'e'
----------------------------------------------------------------------
| OR
----------------------------------------------------------------------
r 'r'
----------------------------------------------------------------------
) end of \2
----------------------------------------------------------------------
( group and capture to \3:
----------------------------------------------------------------------
.* any character except \n (0 or more times
(matching the most amount possible))
----------------------------------------------------------------------
) end of \3
----------------------------------------------------------------------
$ before an optional \n, and the end of the
string
----------------------------------------------------------------------
) end of grouping
----------------------------------------------------------------------
Give a man a fish: <%-{-{-{-<
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