Question:What is the functionality of the function strstr and stristr? 

strstr():
       The strstr() function returns the remainder of a string beginning with the first occurrence of a predefined string.Its prototype follows:            
                   string strstr(string str, string occurrence[,bool before_needle]) 
stristr(): 
       Same as strstr() function except the search of for the pattern in case insensitive.

Question:What is regular expression? 

Regular expression provide the foundation of describing or matching data according to defined syntax rules. A regular expression is nothing more than a pattern of characters itself, matched against a certain parcel of text.

Question:What is the functionality of the function strstr and stristr?

 

Strstr(): The strstr() function returns the remainder of a string beginning with the first occurrence of a predefined string.

Stristr():The strstr() function returns the remainder of a string beginning with the first occurrence of a predefined string ignoring case-sensitivity.