Use Java and RegEx to convert casing in a string

Question

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

Answer 1

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

• StringBuilder and StringBuffer in Java