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javaregexstringuppercaselowercase

Use Java and RegEx to convert casing in a string

Problem: Turn 

"My Testtext TARGETSTRING My Testtext"

into 

"My Testtext targetstring My Testtext"

Perl supports the "\L"-operation which can be used in the replacement-string.

The Pattern-Class does not support this operation:

Perl constructs not supported by this class: [...] The preprocessing operations \l \u, \L, and \U. https://docs.oracle.com/javase/10/docs/api/java/util/regex/Pattern.html

Solution

  • You can't do this in Java regex. You'd have to manually post-process using String.toUpperCase() and toLowerCase() instead.

    Here's an example of how you use regex to find and capitalize words of length at least 3 in a sentence 

        String text = "no way oh my god it cannot be";
    Matcher m = Pattern.compile("\\b\\w{3,}\\b").matcher(text);

    StringBuilder sb = new StringBuilder();
    int last = 0;
    while (m.find()) {
        sb.append(text.substring(last, m.start()));
        sb.append(m.group(0).toUpperCase());
        last = m.end();
    }
    sb.append(text.substring(last));

    System.out.println(sb.toString());
    // prints "no WAY oh my GOD it CANNOT be"

    Note on appendReplacement and appendTail

    Note that the above solution uses substring and manages a tail index, etc. In fact, you can go without these if you use Matcher.appendReplacement and appendTail.

        StringBuffer sb = new StringBuffer();
    while (m.find()) {
        m.appendReplacement(sb, m.group().toUpperCase());
    }
    m.appendTail(sb);

    Note how sb is now a StringBuffer instead of StringBuilder. Until Matcher provides StringBuilder overloads, you're stuck with the slower StringBuffer if you want to use these methods.

    It's up to you whether the trade-off in less efficiency for higher readability is worth it or not.

    See also