StringCount

StringCount["string","sub"] gives a count of the number of times "sub" appears as a substring of "string". 

StringCount["string",patt] gives the number of substrings in "string" that match the general string expression patt. 

StringCount["string",{patt1,patt2,…}] counts the number of occurrences of any of the patti. 

StringCount[{s1,s2,…},p] gives the list of results for each of the si.

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