regex - Putting space in camel case string using regular expression

Question

I am driving my question from add a space between two words.

Requirement: Split a camel case string and put spaces just before the capital letter which is followed by a small case letter or may be nothing. The space should not incur between capital letters.

eg: CSVFilesAreCoolButTXT is a string I want to yield it this way CSV Files Are Cool But TXT

I drove a regular express this way:

"LightPurple".replace(/([a-z])([A-Z])/, '$1 $2')

If you have more than 2 words, then you'll need to use the g flag, to match them all.

"LightPurpleCar".replace(/([a-z])([A-Z])/g, '$1 $2')

If are trying to split words like CSVFile then you might need to use this regexp instead:

"CSVFilesAreCool".replace(/([a-zA-Z])([A-Z])([a-z])/g, '$1 $2$3')

But still it does not serve the way I have put my requirements.

Answer 1

var rex = /([A-Z])([A-Z])([a-z])|([a-z])([A-Z])/g;

"CSVFilesAreCoolButTXT".replace( rex, '$1$4 $2$3$5' );
// "CSV Files Are Cool But TXT"

And also

"CSVFilesAreCoolButTXTRules".replace( rex, '$1$4 $2$3$5' );    
// "CSV Files Are Cool But TXT Rules"

The text of the subject string that matches the regex pattern will be replaced by the replacement string '$1$4 $2$3$5', where the $1, $2 etc. refer to the substrings matched by the pattern's capture groups ().

$1 refers to the substring matched by the first ([A-Z]) sub-pattern, and $3 refers to the substring matched by the first ([a-z]) sub-pattern etc.

Because of the alternation character |, to make a match the regex will have to match either the ([A-Z])([A-Z])([a-z]) sub-pattern or the ([a-z])([A-Z]) sub-pattern, so if a match is made several of the capture groups will remain unmatched. These capture groups can be referenced in the replacement string but they have have no effect upon it - effectively, they will reference an empty string.

The space in the replacement string ensures a space is inserted in the subject string every time a match is made (the trailing g flag means the regular expression engine will look for more than one match).