how can I get the left string at caret pos?

[looker]

I want to get the left string at current caret , like this :

>This is a test

If caret is at begin of "test",I want to get the string between ">" and "test",the result is "This is a ".
How can I do?

[Sergey Tkachenko]

Do you need text from the beginning of the screen line, of from the beginning of the paragraph? If there are some nontext items (pictures) there, should they be ignored, or in this case you need text from the picture to the caret?