In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Infants were assigned to the cocaine - exposed group if there was a maternal history of cocaine abuse during pregnancy or if maternal or neonatal urine toxicology results were positive at the time of delivery .
### Output Text:
Infants were assigned to the <span class="chemical">cocaine</span> - exposed group if there was a maternal history of cocaine abuse during pregnancy or if maternal or neonatal urine toxicology results were positive at the time of delivery . 