How can I use Pattern and Matcher instead of split() to do this?

Question

I have the following function:

@testVisible private String selectBeforeKeyword (String remainingStepsAsString, String keyword)
{
    List<String> splitList = remainingStepsAsString.split('\\b'+keyword+'\\b');

    return (splitList.size() > 1)
                ? (splitList[0].trim())
                : null;
}

This works as I need it to. Yet I am not really looking to split the the string, but rather to capture the beginning of it. So, I'm thinking a more appropriate solution might be something like this:

@testVisible private String selectBeforeKeyword (String remainingStepsAsString, String keyword)
{
    Pattern keywordPattern = Pattern.compile('(.*)\\b'+keyword+'\\b');
    Matcher keywordMatcher = keywordPattern.matcher(remainingStepsAsString);

        return (keywordMatcher.matches())
                ? (keywordMatcher.group(0).trim())
                : null;
}

... however, this doesn't work, returning null instead.

What would the correct pattern to compile to make this work?

Answer 1

I was curious to see whether @Alexander was right, so I set up a simpler pattern matcher, and compared its runtime to the string split method. Each method was executed 1,000 times per run:

String keyword = 'World ';
String myString = 'Hello World.';

Integer stringSplitBegin; 
Integer stringSplitEnd;
Integer patternMatchBegin;
Integer patternMatchEnd;

stringSplitBegin = DateTime.now().millisecond();
for(Integer i=0; i < 1000; i++){
    String keyword = 'World';
    String myString = 'Hello World.';
    List<String> splitList = myString.split('\\b'+keyword+'\\b');

}
stringSplitEnd = DateTime.now().millisecond();


patternMatchBegin = DateTime.now().millisecond();
for(Integer i=0; i < 1000; i++){
    Pattern keywordPattern = Pattern.compile(keyword);
    Matcher keywordMatcher = keywordPattern.matcher(myString);
    
}
patternMatchEnd = DateTime.now().millisecond();

system.debug('stringSplit:  ' + (stringSplitEnd - stringSplitBegin) + 'ms');
system.debug('patternMatch: ' + (patternMatchEnd - patternMatchBegin) + 'ms');

I ran the code 5 times as anonymous Apex, and these were the results:

|DEBUG|stringSplit: 13ms :: patternMatch: 60ms

|DEBUG|stringSplit: 10ms :: patternMatch: 51ms

|DEBUG|stringSplit: 10ms :: patternMatch: 41ms

|DEBUG|stringSplit: 9ms :: patternMatch: 41ms

|DEBUG|stringSplit: 11ms :: patternMatch: 49ms

So as it works out, Alex was right. The string split method was consistently faster, by an average of 37.8ms.

I had been hoping to find a way to get the pattern matcher to work with a positive lookahead, but I don't think Apex supports them. No matter anyway, it seems!

Pattern keywordPattern = Pattern.compile('^.*(?=(' + keyword + '))');