Mention the use of a character class for metacharacters (#687)

Closes #673

strings.Rmd

@@ -299,6 +299,17 @@ There are a number of special patterns that match more than one character. You'v
 
 Remember, to create a regular expression containing `\d` or `\s`, you'll need to escape the `\` for the string, so you'll type `"\\d"` or `"\\s"`.
 
+A character class containing a single character is a nice alternative to backslash escapes when you want to include a single metacharacter in a regex. Many people find this more readable.
+
+```{r}
+# Look for a literal character that normally has special meaning in a regex
+str_view(c("abc", "a.c", "a*c", "a c"), "a[.]c")
+str_view(c("abc", "a.c", "a*c", "a c"), ".[*]c")
+str_view(c("abc", "a.c", "a*c", "a c"), "a[ ]")
+```
+
+This works for most (but not all) regex metacharacters: `$` `.` `|` `?` `*` `+` `(` `)` `[` `{`. Unfortunately, a few characters have special meaning even inside a character class and must be handled with backslash escapes: `]` `\` `^` and `-`.
+
 You can use _alternation_ to pick between one or more alternative patterns. For example, `abc|d..f` will match either '"abc"', or `"deaf"`. Note that the precedence for `|` is low, so that `abc|xyz` matches `abc` or `xyz` not `abcyz` or `abxyz`. Like with mathematical expressions, if precedence ever gets confusing, use parentheses to make it clear what you want:
 
 ```{r}