Simple recorded macro with
1. .MatchWildcards = True
2. \. 'escapes' the end of sentence period since 'dot' can be a RegEx metacharacter
3. ^13 for the paragraph mark; ^10 if new line mark
4. 'Grouping' ( ) around the matching range a-z so that ...
5. ... the space+\1 in Replacement can refer to what was found in the first (only) grouping
Sub Macro3()
Selection.Find.ClearFormatting
Selection.Find.Replacement.ClearFormatting
With Selection.Find
.Text = "\.^13([a-z])"
.Replacement.Text = " \1"
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchAllWordForms = False
.MatchSoundsLike = False
.MatchWildcards = True
End With
Selection.Find.Execute Replace:=wdReplaceAll
End Sub