Regex that extract string of length that is encoded in string

user_453492 0
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I have the following string to parse:

X4IitemX6Nabc123

that is structured as follows:

  • X... marker for 'field identifier'
  • 4... length of item (name), will change according to length of item name
  • I... identifier for item name, must not be extracted, fixed
  • item... value that should be extraced as "name"
  • X... marker for 'field identifier'
  • 6... length of item (name), will change according to length of item name
  • N... identifier for item number, must not be extracted, fixed
  • abc123... value that should be extraced as "num" Only these two values will be contained in the string, the sequence is also always the same (name, nmuber).

What I have so far is

\AX(?I<namelen>\d+)U(?<name>.+)X(?<numlen>\d+)N(?<num>.+)$

But that does not take into account that the length of the name is contained in the string itself. Somehow the .+ in the name group should be replaced by .{4}. I tried {$1}, {${namlen}} but that does not yield the result I expect (on rubular.com or regex.191)

Any ideas or further references?

regex-group regex
1 Answers

What you ask for is only possible in languages that allow code insertions in the regex pattern.

Here is a Perl example:

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

my $text = "X4IitemX6Nabc123";
if ($text =~ m/^X(?<namelen>[0-9]+)I(?<name>(??{".{".$^N."}"}))X(?<numlen>[0-9]+)N(?<num>.+)$/) {
    print $text . ": PASS!\n";
} else {
    print $text . ": FAIL!\n"
}
# -> X4IitemX6Nabc123: PASS!

In other languages, use a two-step approach:

  1. Extract the number after X,
  2. Build a regex dynamically using the result of the first step.

See a JavaScript example:

const text = "X4IitemX6Nabc123";
const rx1 = /^X(\d+)/;
const m1 = rx1.exec(text)
if (m1) {
  const rx2 = new RegExp(`^X(?<namelen>\\d+)I(?<name>.{${m1[1]}})X(?<numlen>\\d+)N(?<num>.+)$`)
  if (rx2.test(text)) {
    console.log(text, '-> MATCH!')
  } else console.log(text, '-> FAIL!');
} else {
  console.log(text, '-> FAIL!')
}

See the Python demo:

import re
text = "X4IitemX6Nabc123"
rx1 = r'^X(\d+)'
m1 = re.search(rx1, text)
if m1:
  rx2 = fr'^X(?P<namelen>\d+)I(?P<name>.{{{m1.group(1)}}})X(?P<numlen>\d+)N(?P<num>.+)$'
  if re.search(rx2, text):
    print(text, '-> MATCH!')
  else:
    print(text, '-> FAIL!')
else:
  print(text, '-> FAIL!')

# => X4IitemX6Nabc123 -> MATCH!
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