GREP search for space before first instance of a character style

I don't know if I understood your ask properly but if it is always a space before the text then you could apply the character style to the space as well and then run the Grep to find the text with the character style applied. The first character of your result would be the space character you wanted. Now if we can't or won't apply the character style to the space character then I don't think we can find it by just using a Grep query. We would need the help of script. Check out the following script

app.findTextPreferences = null
app.findTextPreferences.appliedCharacterStyle = "CS1"
var r = app.documents[0].findText()
for(var i = 0 ; i < r.length; i++){
    var sb = r[i].parentStory.characters[r[i].index - 1]
    sb.contents = SpecialCharacters.RIGHT_INDENT_TAB
}
app.findTextPreferences = null

Edit :- Change CS1 with the name of your style in the code above.

-Manan