private static RPattern pattern()

in src/main/java/org/apache/commons/codec/language/bm/Rule.java [488:556]
55 lines of code
23 McCabe index (conditional complexity)

    private static RPattern pattern(final String regex) {
        final boolean startsWith = regex.startsWith("^");
        final boolean endsWith = regex.endsWith("$");
        final String content = regex.substring(startsWith ? 1 : 0, endsWith ? regex.length() - 1 : regex.length());
        final boolean boxes = content.contains("[");

        if (!boxes) {
            if (startsWith && endsWith) {
                // exact match
                if (content.isEmpty()) {
                    // empty
                    return input -> input.length() == 0;
                }
                return input -> input.equals(content);
            }
            if ((startsWith || endsWith) && content.isEmpty()) {
                // matches every string
                return ALL_STRINGS_RMATCHER;
            }
            if (startsWith) {
                // matches from start
                return input -> startsWith(input, content);
            }
            if (endsWith) {
                // matches from start
                return input -> endsWith(input, content);
            }
        } else {
            final boolean startsWithBox = content.startsWith("[");
            final boolean endsWithBox = content.endsWith("]");

            if (startsWithBox && endsWithBox) {
                String boxContent = content.substring(1, content.length() - 1);
                if (!boxContent.contains("[")) {
                    // box containing alternatives
                    final boolean negate = boxContent.startsWith("^");
                    if (negate) {
                        boxContent = boxContent.substring(1);
                    }
                    final String bContent = boxContent;
                    final boolean shouldMatch = !negate;

                    if (startsWith && endsWith) {
                        // exact match
                        return input -> input.length() == 1 && contains(bContent, input.charAt(0)) == shouldMatch;
                    }
                    if (startsWith) {
                        // first char
                        return input -> input.length() > 0 && contains(bContent, input.charAt(0)) == shouldMatch;
                    }
                    if (endsWith) {
                        // last char
                        return input -> input.length() > 0 &&
                               contains(bContent, input.charAt(input.length() - 1)) == shouldMatch;
                    }
                }
            }
        }

        return new RPattern() {
            final Pattern pattern = Pattern.compile(regex);

            @Override
            public boolean isMatch(final CharSequence input) {
                final Matcher matcher = pattern.matcher(input);
                return matcher.find();
            }
        };
    }