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Regexp Quantifiers

20180608 Quantifiers are used to match repitition of a pattern within a string. The following table shows a list of quantifiers.

	Quantifier	Description
1	*	The preceeding item is matched 0 or more times
2	+	The preceeding item is matched 1 or more times
3	?	The preceeding item is matched at most 1 times.
4	{n}	The preceeding item is matched n times.
5	{n,}	The preceeding item is matched atleast n times.

Some examples will illustrate.

s <- c("aaab", "abb", "bc", "abbcd", "bbbc", "abab", "caa")
grep(pattern="ab*b", s, value=TRUE)

## [1] "aaab"  "abb"   "abbcd" "abab"

grep(pattern="abbc?", s, value=TRUE)

## [1] "abb"   "abbcd"

grep(pattern="b{2,}?", s, value=TRUE)

## [1] "abb"   "abbcd" "bbbc"