How to set a regular expression for the first letter in a sentence?

Question

Regular Expressions

I have a sentence
this text has two sentences. good fortune.

It is necessary through a regular expression to capture the first letter of the word this and the first letter of the word good in each sentence (it is guaranteed that the letters are lower case)

Answer 1

/^[az]|(?<=\.\s*)[az]/gm
if the dialect does not allow, can be divided into groups
/^([a-z])|\.\s*([a-z])/gm

Answer 2

/(?<=^|\.)\s?([a-z])/g