My spec states that a valid account number is 20 characters long and consists of at least one digit followed by spaces (and just spaces, not everything that matches \s). The account number is extracted from a longer string.

I am using Parse::RecDescent to validate the longer string as 

record  : account address info
account : /trusty regex here/
address : ...
info    : ...

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There is a lot more to the grammar (about another 200 lines), but this is the bit relevant to my problem.

Everything in record is fixed length.

But I am having trouble writing a one-shot regex to validate this.

I think I want to do this 

account : /\d(\d){0,19} {19 - length \1}/

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but that doesn't work. I definitely want the account rule to consume 20 characters, so that the address rule starts at the right point. 
 01234567890123456789
'123                 ' # OK
'123 123             ' # NOK
'                    ' # NOK
'123c                ' # NOK

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Am I being dense or is this tricky ?

+++++++++++++++++
#!/usr/bin/perl
use warnings;use strict;use brain;

In reply to Regex to match 20 chars of some digits followed by some spaces by leriksen

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