Match several occurrences or zero (in this order) using regular expressions

Question

Hello all,

I want to match roman numbers using Groovy regular expressions (I have not tried this in Java but should be the same). I found an answer in this website in which someone suggested the following regex:

/M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})/

The problem is that a expression like /V?I{0,3}/ is not greedy in Groovy. So for a string like "Book number VII" the matcher /V?I{0,3}/ returns "V" and not "VII" as it would be desired.

Obviously if we use the pattern /VI+/ then we DO get the match "VII"... but this solution is not valid if the string is something like "Book number V" as we will get no matches...

I tried to force the maximum character catching by using a greedy quantifier /VI{0,3}+/ or even /VI*+/ but I still get the match "V" over "VII"

Any ideas?

With best regards. Alex

Answer 1

Why not just (IX|IV|V?I{1,3}|V) ?

Answer 2

I found what my mistake was. Thing is that patterns like /V?I{0,3}/ or /V?I*/ are met even by EMPTY strings... so for a string like "Book VII" the matcher will throw the following result matches:

Result[0] --> ''
Result[1] --> '' 
Result[2] --> ''
Result[3] --> '' 
Result[4] --> '' 
Result[5] --> 'VII'
Result[6] --> '' 

The greedy result is there (Result[5]) alright. My problem was that I was always picking the first match (Result[0]) and that is only valid if the pattern is not met by empty strings.

For instance, the suggested pattern /V?I{1,3}|V/ will throw only one result, so picking the first result match is Ok:

Result[0] --> 'VII'

... This is so since the pattern is not met by empty strings.

Hope this helps others