Question

Why this code:

String keyword = "pattern";
String text    = "sometextpatternsometext";
String patternStr = "^.*" + keyword + ".*$"; //
Pattern pattern = Pattern.compile(patternStr, Pattern.CASE_INSENSITIVE);
Matcher matcher = pattern.matcher(text);
while (matcher.find()) {
    int start = matcher.start();
    int end = matcher.end();
    System.out.println("start = " + start + ", end = " + end);
}

start = 0, end = 23

don’t work properly.

But, this code:

String keyword = "pattern";
String text    = "sometext pattern sometext";
String patternStr = "\\b" + keyword + "\\b"; //
Pattern pattern = Pattern.compile(patternStr, Pattern.CASE_INSENSITIVE);
Matcher matcher = pattern.matcher(text);
while (matcher.find()) {
    int start = matcher.start();
    int end = matcher.end();
    System.out.println("start = " + start + ", end = " + end);
}

start = 9, end = 16

work fine.

Answer 1

It does work. Your pattern

^.*pattern.*$

says to match:

1. start at the beginning
2. accept any number of characters
3. followed by the string pattern
4. followed by any number of characters
5. until the end of the string

The result is the entire input string. If you wanted to find only the word pattern, then the regex would be just the word by itself, or as you found, bracketed with word-boundary metacharacters.