SOURCERECTATTIME FOR STRING

It IS possible to measure parts of a text layer using text animators and some time-based trickery. If you use a Character Value animator to change the characters you don't want to measure into whitespace characters, sourceRectAtTime() will only measure the bounding box of the characters that remain unchanged. Now, obviously this makes part of your text invisible, but the important thing to remember is you can measure the sourceRect at any time, including times your text isn't onscreen, even including times outside the running time of your comp.

I typically do this by having the Character Value animator only blank out the extraneous characters at a negative time value, i.e. before the start of the comp. Then I feed that time into sourceRectAtTime() and it gives me just the dimensions of just the characters I want to measure.  I use this trick all the time to do things like measuring individal lines, words, or characters, and theres no reason it wouldn't work to measure a particular phrase.

If this sounds like something you'd like help working out in more detail, please let me know.

As Mylenium says, this is only possible, if you set the text of a layer to exactly the text whose size you want to measure. If your goal is to split the text into these parts, the API of TextExploder could be an option:

https://mamoworld.com/article/textexploder-scripting-api

This is not directly possible. The text will have to be treated with string operations and the processet output piped into a second text layer with identical formatting. From that you can glean all the info you need and piece the text together by calculating positions and that. Since it's convoluted and never actually looks the same due to typographical calculations needing to consider a ton of factors for rendering text it would be best if you just structured your text suitably from the outset and format it manually.

Mylenium