How does the dot metacharacter match newline characters?

Question

I thought that the dot . in regex will match any character, except the end-of-line character.

However, in R, I found that the dot can match anything, including the newline characters \n, \r or \r\n:

grep(c("\r","\n","\r\n"),pattern=".")
[1] 1 2 3

Can someone explain the contradiction?

Answer 1

The page here http://www.regular-expressions.info/dot.html explains how the rule that dot does not match the end-of-line character exists mostly for historic reasons:

The first tools that used regular expressions were line-based. They would read a file line by line, and apply the regular expression separately to each line. The effect is that with these tools, the string could never contain line breaks, so the dot could never match them.

However,

Modern tools and languages can apply regular expressions to very large strings or even entire files. Except for JavaScript and VBScript, all regex flavors discussed here have an option to make the dot match all characters, including line breaks.

Apparently, R is one such language where by default, dot will match every character. (I point you to Joshua's comment above, recommending you look at ?regex and the POSIX 1003.2 standard.)

---

The page I linked above also mentions Perl and suggests how under its default mode, dot will not match line breaks.

Notice how R's grep function has a perl option. If you turn it on, you do get a different output:

> grep(".", c("\r","\n","\r\n"), perl = TRUE)
[1] 1 3

This is telling me that \n is the line break character, but not \r. Something that comparing cat("\r") and cat("\n") can confirm.

(I'm on a Mac OS if it makes any difference.)