Could anyone help me in understanding the groovy script below

Question

I am trying to decode some groovy script. I was able to figure out that it is a regular expression but couldn't figure out what the code is exactly.

def dirNumber = this.'Directory Number'
dirNumber?"61" + (dirNumber =~ /0([0-9]+)/)[0][1] + "@":null

Answer 1

According to Regular expression operators section of https://groovy-lang.org/operators.html, =~ is the find operator, which creates a java.util.regex.Matcher for pattern to the right matching them on string to the left. So, dirNumber =~ /0([0-9]+)/ is equivalent to Pattern.compile("/0([0-9]+)/").matcher(dirNumber) and evaluates to an instance of java.util.regex.Matcher.

Groovy gives you the ability to access matches by index ([0] in your code); your regular expression uses grouping, so in each match you can access groups by (1-based: 0 denotes the entire pattern) index ([1] in your code), too.

So, your code (if dirNumber is not null) extracts the first group of the first match of the regular expression.

// EDITED

You can get an IndexOutOfBoundsException when the first index ([0] in your code) is out of the matches' range; when your second index ([1] in you code) is of the grous' range, you get a null without exception accessing the group through index...

You can test these scenarios with a simplified version of the code above:

def dirNumber = "Xxx 15 Xxx 16 Xxx 17"
def matcher = dirNumber =~ /Xxx ([0-9]+)/
println(matcher[0][1]) // 15
println(matcher[0][5]) // null 
println(matcher[5][1]) // IndexOutOfBoundsException