table regular expression assertions symbo meaning matches at the startalso matches after each newline with the re multiline flag matches at the endalso matches before each newline with the re multiline flag \ matches at the start \ \ matches at "wordboundaryinfluenced by the re ascii flag--inside character class this is the escape for the backspace character matches at non-"wordboundaryinfluenced by the re ascii flag \ matches at the end (?=ematches if the expression matches at this assertion but does not advance over it--called lookahead or positive lookahead (?!ematches if the expression does not match at this assertion and does not advance over it--called negative lookahead (?<=ematches if the expression matches immediately before this assertion--called positive lookbehind (?<!ematches if the expression does not match immediately before this assertion--called negative lookbehind raw strings in the context of python program we would normally write regex like this inside raw triple quoted string--raw so that we don' have to double up the backslashesand triple quoted so that we can spread it over multiple lines in addition to the assertions we have discussed so farthere are additional assertions which look at the text in front of (or behindthe assertion to see whether it matches (or does not matchan expression we specify the expressions that can be used in lookbehind assertions must be of fixed length (so the quantifiers ?+and cannot be usedand numeric quantifiers must be of fixed sizefor example{ }in the case of the key=value regexthe negative lookbehind assertion means that at the point it occurs the preceding character must not be space or tab this has the effect of ensuring that the last character captured into the "valuecapture group is not space or tab (yet without preventing spaces or tabs from appearing inside the captured textlet' consider another example suppose we are reading multiline text that contains the names "helen patricia sharman""jim sharman""sharman joshi""helen kelly"and so onand we want to match "helen patricia"but only when referring to "helen patricia sharmanthe easiregex flags