Processing different matches in a RegExp

Question

I want to construct a RegExp that matches either the first letter of the first word, or the first letter of non-first words, but I want to be able to distinguish between these two kinds matches so I can process non-first-word matches differently.

For example I want to always capitalise the first word and all other words apart from instances of "bam" that are not the first word.

How can I do this?

function titleCase(title) {
    return title.replace(/\b(.)/g, (str, p1) => {         
         return p1.toUpperCase();
    });
}

titleCase('foo bar bam'); // Foo Bar Bam - but I want Foo Bar bam

Answer 1

You can put multiple regex pieces together like /(option 1)|(option 2)/g

Then your callback will receive either of:

callback(match, option1, undefined, offset, subject) // left side matched
callback(match, undefined, option2, offset, subject) // right side matched

Using this, your callback can look like:

(str, p1, p2) => {
    if( p1) ...
    else if( p2) ...
}

Answer 2

Here is an implementation of titleCase which will capitalize the first letter of every word-like element in the string.

function titleCase(title) {
  return title.replace(/\b(\w)(.*?)\b/g, (match, g1, g2) => `${g1.toUpperCase()}${g2}`)
}

1. In this example, /\\b(\\w)(.*?)\\b/g will match a word boundary ( \\b ) followed by a word character ( \\w ). We capture the first letter using the grouping (\\w) , and is used as g1 in the replace function.
2. Then we capture the remaining portion of the word using the group (.*?) , which is referenced using g2 in the replacer.
3. Finally, a template string is used to capitalize the first letter group and concatenate the result with the remaining match of the word.