Java regex repeating capture groups

Question

Considering the following string: "${test.one}${test.two}" I would like my regex to return two matches, namely "test.one" and "test.two". To do that I have the following snippet:

import java.util.regex.Matcher; import java.util.regex.Pattern;

public class RegexTester {

    private static final Pattern pattern = Pattern.compile("\\$\\{((?:(?:[A-z]+(?:\\.[A-z0-9()\\[\\]\"]+)*)+|(?:\"[\\w/?.&=_\\-]*\")+)+)}+$");

    public static void main(String[] args) {
        String testString = "${test.one}${test.two}";

        Matcher matcher = pattern.matcher(testString);

        while (matcher.find()) {
            for (int i = 0; i <= matcher.groupCount(); i++) {
                System.out.println(matcher.group(i));
            }
        }
    }
}

I have some other stuff in there as well, because I want this to also be a valid match ${test.one}${"hello"}.

So, basically, I just want it to match on anything inside of ${} as long as it either follows the format: something.somethingelse (alphanumeric only there) or something.somethingElse() or "something inside of quotations" (alphanumeric plus some other characters). I have the main regex working, or so I think, but when I run the code, it finds two groups,

${test.two} test.two

I want the output to be

test.one test.two

Something like `\$\{(\"[^\"]*\"|\w+(?:)?(?:\.\w+(?:)?)*)}`? See https://regex101.com/r/ILmyTj/1 — Wiktor Stribiżew, May 01 '20 at 17:40
And ignore group zero, which represents the entire expression. — andrewJames, May 01 '20 at 17:43
I expect you meant `${test.one} test.two` for your third-to-last line. — Cary Swoveland, May 01 '20 at 18:25
@WiktorStribiżew - that basically works. I tweaked it a little. So, what was the issue with the regex that I had? I'd just like to understand what I was doing wrong so I can hopefully not repeat the same mistake :) — cloudwalker, May 01 '20 at 18:46
@cloudwalker I tried to explain, but your regex is too cumbersome. If you need more drill-through, please let me know. — Wiktor Stribiżew, May 01 '20 at 19:20
Given the three acceptable formats you have given I assume you do not wish to match `${te.st.one}`, `${testone}` or `${"hel.lo"}`. Is that correct? — Cary Swoveland, May 01 '20 at 21:43

score 2 · Accepted Answer · answered May 01 '20 at 19:16

Basically, your regex main problem is that it matches only at the end of string, and you match many more chars that just letters with [A-z]. Your grouping also seem off.

If you load your regex at regex101, you will see it matches

\$\{
( - start of a capturing group
- (?: - start of a non-capturing group
  - (?:[A-z]+ - start of a non-capturing group, and it matches 1+ chars between A and z (your first mistake)
    - (?:\.[A-z0-9()\[\]\"]+)* - 0 or more repetitions of a . and then 1+ letters, digits, (, ), [, ], ", \, ^, _, and a backtick
  - )+ - repeat the non-capturing group 1 or more times
  - | - or
  - (?:\"[\w/?.&=_\-]*\")+ - 1 or more occurrences of ", 0 or more word, /, ?, ., &, =, _, - chars and then a "
  - )+ - repeat the group pattern 1+ times
- ) - end of non-capturing group
}+ - 1+ } chars
$ - end of string.

To match any occurrence of your pattern inside a string, you need to use

\$\{(\"[^\"]*\"|\w+(?:\(\))?(?:\.\w+(?:\(\))?)*)}

See the regex demo, get Group 1 value after a match is found. Details:

\$\{ - a ${ substring
(\"[^\"]*\"|\w+(?:)?(?:\.\w+(?:)?)*) - Capturing group 1:
- \"[^\"]*\" - ", 0+ chars other than " and then a "
- | - or
- \w+(?:)? - 1+ word chars and an optional () substring
- (?:\.\w+(?:)?)* - 0 or more repetitions of . and then 1+ word chars and an optional () substring
} - a } char.

See the Java demo:

String s = "${test.one}${test.two}\n${test.one}${test.two()}\n${test.one}${\"hello\"}";
Pattern pattern = Pattern.compile("\\$\\{(\"[^\"]*\"|\\w+(?:\\(\\))?(?:\\.\\w+(?:\\(\\))?)*)}");
Matcher matcher = pattern.matcher(s);
while (matcher.find()){
    System.out.println(matcher.group(1)); 
}

Output:

test.one
test.two
test.one
test.two()
test.one
"hello"

Thank you, that was super helpful! I've been trying to get into regex stuff more here lately as it is something that I do not have a ton of experience with in the past, and it's definitely been a rocky start :) — cloudwalker, May 01 '20 at 19:29

score 0 · Answer 2 · answered May 01 '20 at 21:30

You could use the regular expression

(?<=\$\{")[a-z]+(?="\})|(?<=\$\{)[a-z]+\.[a-z]+(?:\(\))?(?=\})

which has no capture groups. The characters classes [a-z] can be modified as required provided they do not include a double-quote, period or right brace.

Demo

Java's regex engine performs the following operations.

(?<=\$\{")  # match '${"' in a positive lookbehind
[a-z]+      # match 1+ lowercase letters 
(?="\})     # match '"}' in a positive lookahead
|           # or 
(?<=\$\{)   # match '${' in a positive lookbehind
[a-z]+      # match 1+ lowercase letters 
\.[a-z]+    # match '.' followed by 1+ lowercase letters
(?:\(\))?   # optionally match `()`
(?=\})      # match '}' in a positive lookahead

Java regex repeating capture groups

2 Answers2