[BF] valid participant

[arnaudbore]

Fix #212

[codecov-commenter]

Codecov Report

Merging #215 (6b79575) into master (3c5ae96) will decrease coverage by 0.18%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   78.30%   78.12%   -0.18%     
==========================================
  Files          15       15              
  Lines         885      887       +2     
  Branches      169      170       +1     
==========================================
  Hits          693      693              
- Misses        159      160       +1     
- Partials       33       34       +1     

Flag	Coverage Δ
pytest	78.12% <0.00%> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dcm2bids/participant.py	95.12% <0.00%> (-4.88%)	⬇️

[SamGuay]

Well thought to allow only alphanum 😄