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: RESULTS : The bupivacaine / rofecoxib group reported significantly less pain , as assessed by a visual analog scale , compared with the other three treatment groups over the first 4 h .
### Output Text:
RESULTS : The <span class="chemical">bupivacaine</span> / <span class="chemical">rofecoxib</span> group reported significantly less pain , as assessed by a visual analog scale , compared with the other three treatment groups over the first 4 h . 