Java Regex to match Dockerfile arg values

Question

I'm trying to extract Dockerfile ARG values which are usually of these forms:

$foo, expected output: foo
${foo}, expected output: foo

I want to capture the string after $. I'm trying this regex right now but it doesn't seem to working for second test case with curly braces:

private static String extract(String input) {
    Pattern argPattern = Pattern.compile("^\\$([^{}]+)$");
    Matcher matcher = argPattern.matcher(input);
    if (matcher.find()) {
        return (matcher.group(1));
    }
    return null;
}

Could anyone please tell me what I'm missing here?

What kind of chars can there be in between { and }? Only letters, digits, _ or more? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 28, 2020 at 19:59
Ok, then [^{}] in my solution below can be replaced with another \w. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Sep 28, 2020 at 20:06

Wiktor Stribiżew · Accepted Answer · 2020-09-28 20:03:14Z

1

You can use

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

See the regex demo. Details:

\$ - a $ char
(?:\{([^{}]+)\}|(\w+)) - either of the two alternatives:
- \{([^{}]+)\} - {, then Group 1 capturing one or more chars other than { and } and then a }
- | - or
- (\w+) - Group 2: any one or more letters, digits or underscores.

See a Java demo:

String s = "$foo1 expected output: foo ${foo2}, expected output: foo2";
Pattern pattern = Pattern.compile("\\$(?:\\{([^{}]+)\\}|(\\w+))");
Matcher matcher = pattern.matcher(s);
while (matcher.find()){
    if (matcher.group(1) != null) {
        System.out.println(matcher.group(1)); 
    } else { 
        System.out.println(matcher.group(2)); 
    }
}

Output: foo1 and foo2.

answered Sep 28, 2020 at 20:03

Wiktor Stribiżew

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1 Comment

Rohan Kumar Over a year ago

Thanks a lot for your response!

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