Fix BST parsing for latex math symbols #11338

@InProceedings{Sadeghifar2014,
  author           = {Mohammad Reza Sadeghifar and J. Jacob Wikner and Oscar Gustafsson},
  booktitle        = {IEEE International Symposium on Circuits and Systemss, ISCAS 2014, Melbourne, Victoria, Australia, June 1-5, 2014},
  title            = {Linear programming design of semi-digital FIR filter and {{$\Sigma$}}{{$\Delta$}} modulator for VDSL2 transmitter},
  year             = {2014},
  month            = jun,
  pages            = {2465--2468},
  publisher        = {{IEEE}},
  bibsource        = {dblp computer science bibliography, http://dblp.org},
  biburl           = {http://dblp.uni-trier.de/rec/bib/conf/iscas/SadeghifarWG14},
  creationdate     = {2024-05-20T11:28:55},
  doi              = {10.1109/ISCAS.2014.6865672},
  modificationdate = {2024-05-20T11:36:27},
  timestamp        = {Fri, 20 May 2016 09:19:46 +0200},
  url              = {http://dx.doi.org/10.1109/ISCAS.2014.6865672},
}