Java Regex : Find out nested patterns and repeated occurrences [duplicate]

Question

I am trying to parse String that has the following patterns:

a2[u]
3[rst]5[g]
3[r2[g]]

I want to extract these strings into following tokens:

2 [u]
3 [rst], 5 [g]
2 [r, 3 [r2[g]] (nested groups)

I am using the following Pattern and Code:

Pattern MY_PATTERN = Pattern.compile("(\\d+)\\[(.+)\\]");
String input = "3[rst]5[g]";
Matcher m = MY_PATTERN.matcher(input);
while(m.find()) {
    System.out.println(m.group(1) + " " + m.group(2));
}

However, it matches to last occurrence of ] instead of the first and that results in an unexpected results. If I change the pattern to (\\d+)\\[(\\w+)\\], it works but fails for 3[r2[g]]. What changes do I need to make so that it doesn't count the whole string as one match?

If you plan to match more nested levels than 1, the regex will become unwieldly. Else, use "(\\d+)\\[([^\\]\\[]*(?:\\[[^\\]\\[]*][^\\]\\[]*)*)]". — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Nov 19, 2020 at 19:57
@Darshan: I would suggest against using regex for this. Better you use a token parser since you are dealing nested brackets. — anubhava
– anubhava, Commented Nov 20, 2020 at 5:13
@WiktorStribiżew It would need more nested levels I am afraid. So, I would go with a token parser as anubhava suggested. — Darshan Mehta
– Darshan Mehta, Commented Nov 20, 2020 at 8:44

Michael Wiles · Accepted Answer · 2020-11-19 20:15:01Z

-1

Looks like you need to add a quantifier to the .+

As it stands the . will eat the whole string and then only match on the last ]. Add a reluctant quantifier ? to the .+ so make the regex (\\d+)\\[(.+?)\\] and see how far you get...

answered Nov 19, 2020 at 20:15

Michael Wiles

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2 Comments

Wiktor Stribiżew Over a year ago

No way, lazy dot will match 3[r2[g] in 3[r2[g]].

Michael Wiles Over a year ago

okay - so regex is not the answer then

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