Grep will not help here, a script might. An alternative would be to export the book as Tagged Text and to modify the numbers to produce something like this :
<ASCII-MAC>
<vsn:13><fset:InDesign-Roman><ctable:=<Black:COLOR:CMYK:Process:0,0,0,1><Hyperlink:COLOR:CMYK:Process:0.86,0.57,0,0.16>>
<dcs:Hyperlink=<Nextstyle:Hyperlink><cc:Hyperlink><cu:1>>
<dps:NormalParagraphStyle=<Nextstyle:NormalParagraphStyle>>
<pstyle:NormalParagraphStyle><pshadc:Black><pshadt:20.000000>This is line <cstyle:Hyperlink><Hpl:=<HplName:1><HplDest:2><DestKey:1><CharStyleRef:Hyperlink><HplLen:1><HplOff:0><Hid:0><Brdrv:0><Brdrw:Thin><Brdrh:None><Brdrs:Solid><Brdrc:0\,0\,0>>1<cstyle:>.
<pshadc:><pshadt:><pstyle:NormalParagraphStyle><pshadc:Black><pshadt:20.000000>
<pshadc:><pshadt:><pstyle:NormalParagraphStyle><pshadc:Black><pshadt:20.000000>And this is line 2.<pshadc:><pshadt:><HplDestDfn:=<HplDestName:2><DestKey:1><HplDestIdx:34><IsPara:0><Hid:0>>
This is the non-verbose version of Tagged Text as exported by ID. Most of the codes above serve no purpose, were I writing from scratch I would just include pstyle, cstyle and HplDestDfn tags.
AB