Java Regex: Text extraction into an array list

Question

I'm struggling with a simple regex, for which I can't seem to get right.

I have some text like so:

This comment is great **[@madeUpUser1](/madeUpUser1)** You said something similar did you mate? **[@madeUpUser2](/madeUpUser2)**

What I would like to end up with is an array list containing the usernames inbetween the parentheses i.e.:

0.madeUpUser1
1.madeUpUser2

And here is the code I have so far:

List<String> matches = Pattern.compile("\\((.+?)\\)")
        .matcher("This comment is great **[@madeUpUser1](/madeUpUser1)** You said something similar did you mate? **[@madeUpUser2](/madeUpUser2)**")
        .results()
        .map(MatchResult::group)
        .collect(Collectors.toList());

However what I'm getting back is this:

0."(/madeUpUser1)"
1."(/madeUpUser2)"

Again, where I want:

0.madeUpUser1
1.madeUpUser2

i.e. without the parentheses and without the forwardslash

Can anyone shed any light on what I'm doing wrong with my regex please?

I suggest adjusting the title of the question. So far, the accepted answer has no solution for "Text extraction into an array list", only a regex that can be used to do that. Maybe it should sound as "Extraction of string between parentheses excluding the first underscore", or something similar. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jan 7, 2022 at 1:00

Gurmanjot Singh · Accepted Answer · 2022-01-05 13:54:54Z

1

Try this regex:

(?<=\\(/)[^)]+(?=\\))

Click for Demo

Explanation

(?<=\\(/) - positive lookbehind to make sure that the current position is preceded by a (/
[^)]+ - matches 1 or more occurences(as many as possible) of any character that is not a )
(?=\\)) - positive lookahead to make sure that the current position is followed by a )

With the regex you are using, \\((.+?)\\), the following happens:

\\( - matches the opening parenthesis (
(.+?) - matches any character(except a new line character) 1 or more times, as few as possible. This subpattern will keep on expanding the match until it reaches the ). That's why it is matching everything between the parenthesis(even the /)
\\) - matches the closing parenthesis )

edited Jan 5, 2022 at 13:54

answered Jan 5, 2022 at 13:25

Gurmanjot Singh

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2 Comments

MetaCoder Over a year ago

Wow thank you for the fast response, this works as expected. May I be a pain and ask can you explain it a bit for me so I can understand why it works? once again thank you very much!!

Gurmanjot Singh Over a year ago

Performance-wise, Wiktor's answer is more efficient. I made use of lookarounds which are more expensive.

Wiktor Stribiżew · Accepted Answer · 2022-01-05 13:42:53Z

You can match ](/ and then capture any zero or more chars other than ( and ) till the next ), and collect Group 1 matches only:

import java.util.*;
import java.util.regex.*;
import java.util.stream.Collectors;


class Test
{
    public static void main (String[] args) throws java.lang.Exception
    {
        String text = "This comment is great **[@madeUpUser1](/madeUpUser1)** You said something similar did you mate? **[@madeUpUser2](/madeUpUser2)**";

        Pattern p = Pattern.compile("]\\(/([^()]*)\\)");
        List<String> results = p.matcher(text)
            .results()
            .map(mr -> mr.group(1))
            .collect(Collectors.toList());
        
        // Or, to get a string array:
        // String[] results = p.matcher(text).results().map(mr -> mr.group(1)).toArray(String[]::new);

        for (String x: results) {
            System.out.println(x);
        }
    }
}

See the online demo. Output:

madeUpUser1
madeUpUser2

See the regex demo. Details:

]\(/ - a ])/ string
([^()]*) - Capturing group 1: any zero or more chars other than ) and (
\) - a ) char.

The fourth bird · Accepted Answer · 2022-01-05 13:49:19Z

You can use a capture group, and match the outer parenthesis/square brackets:

\(/([^\s()]+)\)

\(/ Match (/
( Capture group 1
- [^\s()]+ Match 1+ chars other than a whitespace char or ( )
) Close group 1
\) Match )

Regex demo

List<String> matches = Pattern.compile("\\(/([^\\s()]+)\\)")
    .matcher("This comment is great **[@madeUpUser1](/madeUpUser1)** You said something similar did you mate? **[@madeUpUser2](/madeUpUser2)**")
    .results()
    .map(m -> m.group(1))
    .collect(Collectors.toList());

for (String s : matches)
    System.out.println(s);

Output

madeUpUser1
madeUpUser2

Or in the example, the string between the square brackets seems to be the same, so another option using the same code could be:

\[@([^\s\]\[]+)]

\[@ match [@
( Capture group 1
- [^\s\]\[]+ Match 1+ chars other than a whitespace char or [ ]
) Close group 1
] Match ]

Regex demo | Java demo

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