TraceScan

TraceScan[f,expr] applies f to all expressions used in the evaluation of expr. 

TraceScan[f,expr,form] includes only those expressions which match form. 

TraceScan[f,expr,s] includes all evaluations which use transformation rules associated with the symbol s. 

TraceScan[f,expr,form,fp] applies f before evaluation and fp after evaluation to expressions used in the evaluation of expr. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.