Makes it possible to easily loop through all of the instances of a pattern within a string and replace the matches with something different.

PatternReplacer.cls

/**
 * Author:  Chris West
 * Description:
 *   Makes it possible to easily loop through all of the instances of a pattern
 *   within a string and replace the matches with something different.
 */
public class PatternReplacer {
    public Matcher matcher {get; private set;}
    public String match {
        get { return p_match; }
        set {
            Integer startIndex = matcher.start() + offset;
            io = io.substring(0, startIndex)
                + value
                + io.substring(startIndex + p_match.length());
            nextOffset += value.length() - p_match.length();
            p_match = value;
        }
    }
    private String p_match;
    private String io;
    private Integer offset;
    private Integer nextOffset = 0;

    public PatternReplacer(String strPattern, String strInput) {
        this(Pattern.compile(strPattern), strInput);
    }

    public PatternReplacer(Pattern pat, String strInput) {
        matcher = pat.matcher(strInput);
        io = strInput;
    }

    public Boolean find() {
        offset = nextOffset;
        Boolean result = matcher.find();
        if (result) {
            p_match = matcher.group();
        }
        return result;
    }

    public override String toString() {
        return io;
    }
}

~example-camel-to-underscore-case.cls

// Turn camel-casing into underscore casing.
PatternReplacer pr = new PatternReplacer('[A-Z]+', 'SpanishFirstName');
while (pr.find()) {
    Matcher m = pr.matcher;
    pr.match = (m.start() == 0 ? '' : '_') + m.group().toLowerCase();
}
System.debug(pr.toString());

~example-count-spaces.cls

// Number the whitespace groups.
PatternReplacer pr = new PatternReplacer('\\s+', 'Your Mac will sleep soon unless     plugged into a power outlet.');
for (Integer n = 1; pr.find(); n++) {
    Matcher m = pr.matcher;
    pr.match = ' (' + n + ') ';
}
System.debug(pr);

~example-reverse-case.cls

// Reverse the casing of normal English letters.
PatternReplacer pr = new PatternReplacer('([A-Z]+)|[a-z]+', 'Where in the world is Carmen Sandiego?');
while (pr.find()) {
    Matcher m = pr.matcher;
    String g = m.group();
    pr.match = (m.group(1) == null ? g.toUpperCase() : g.toLowerCase());
}
System.debug(pr.toString());