Mention quantifier term

Fixes hadley#898

strings.Rmd

@@ -373,7 +373,7 @@ str_view(c("grey", "gray"), "gr(e|a)y")
 
 5.  Create a regular expression that will match telephone numbers as commonly written in your country.
 
-### Repetition
+### Repetition / Quantifiers
 
 The next step up in power involves controlling how many times a pattern matches:
 
@@ -414,6 +414,8 @@ str_view(x, 'C{2,3}?')
 str_view(x, 'C[LX]+?')
 ```
 
+Collectively, these operators are called **quantifiers** because they quantify how many times a match can occur.
+
 #### Exercises
 
 1.  Describe the equivalents of `?`, `+`, `*` in `{m,n}` form.