How to catch roman numbers inside string?

perl, regex

Solution

This is where Perl lets us down with its missing `\<` and `\>` (beginning and end word boundary) constructs that are available elsewhere. A pattern like `\b...\b` will match even if the `...` consumes none of the target string because the second `\b` will happily match the beginning word boundary a second time.

However an end word boundary is just `(?<=\w)(?!\w)` so we can use this instead.

This program will do what you want. It does a look-ahead for a string of potential Roman characters enclosed in word boundaries (so we must be at a beginning word boundary) and then checks for a legal Roman number that isn't followed by a word character (so now we're at an end word boundary).

Note that I've reversed your `>...<` marks as they were confusing me.

use strict;
use warnings;

use feature 'say';

my $x = ' some text I-LXIII iv more ';

if ( $x =~ s{
    (?= \b [CLXVI]+ \b )
    (
      (?:XC|XL|L?X{0,3})?
      (?:IX|IV|V?I{0,3})?
    )
    (?!\w)
    }
    {<$1>}xgi ) {

    say $x;
}

output

some text <I>-<LXIII> <iv> more

Problem

I want to catch roman numbers inside string (numbers below 80 is fine enough). I found good base for it in How do you match only valid roman numerals with a regular expression?. Problem is: it deals with whole strings. I did not found yet a solution how to detect roman numbers inside string, because there is nothing mandatory, every group may be optional. So far i tried something like this: ``` my $x = ' some text I-LXIII iv more '; if ( $x =~ s/\b( ( (XC|XL|L?X{0,3}) # first group 10-90 | (IX|IV|V?I{0,3}) # second group 1-9 )+ ) \b/>$1</xgi ) { # mark every occurrence say $x; } __END__ ><some>< ><text>< ><>I<><-><>LXIII<>< ><>iv<>< ><more>< desired output: some text >I<->LXIII< >iv< more ``` So, this one captures word boundaries by themself too, because all groups are optional. How to get it done? How to make one of those 2 groups mandatory while there is no possible to tell which one is mandatory? Other approaches to catch romans are welcome too.

Original source

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