You can take advanntage of the fixed width property with unpack and validate the data afterwards. 

# Given $record
my %record;
@record{ qw/account address info/ }
    = unpack 'A20 A42 A255', $record; # adjust widths to suit

# ($record{'account'}) = $record{'account'} =~ /^(\d[\d ]*)$/ 
($record{'account'}) = $record{'account'} =~ /^(\d+)$/ 
    or die 'Bad Account ID';    # detaints, too
# verify the rest

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The unpack width enforces the field width you expect. If spaces can't occur between digits, it becomes even simpler. The matching regex would then be /^(\d+)$/. 'An' is the unpack template for a space-padded field of bytes and results in stripping the trailing spaces. In the regex, [\d ] is a character class of digits and spaces.

Update: Simplified the code to agree with leriksen's spec.

After Compline,
Zaxo

In reply to Re: Regex to match 20 chars of some digits followed by some spaces by Zaxo
in thread Regex to match 20 chars of some digits followed by some spaces by leriksen

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