• a pattern that is searched with a certain flag for a part of the pattern and that which has no flags in the pattern • to sub-group the pattern and effectively use flags of (?m) as /m for mult-line strings, (?s) as /s for single line strings, (?x) as /x for extended search including comment lines and space delimiters and (?i) as /i for case sensitive pattern notation or a combination of the flags