空行的正则表达式是什么？

Question

http://www.endmemo.com/program/R/grep.php

I've looked through the chart for a regular expression that corresponds to a blank line, but I just can't seem to find it. I've tried \\\\n but that means a newline, not that it's necessarily blank.

example :

string = c("Precedence: bulk", 
  "List-Unsubscribe: <mailto:zzzzteana-unsubscribe@yahoogroups.com>", 
  "Content-Transfer-Encoding: 7bit", 
  "", 
  "Martin A posted:", 
  "Tassos Papadopoulos, the Greek sculptor behind the plan, judged that the", 
  " Mount Athos monastic community, was ideal for the patriotic sculpture. ", 
  " ", 
  " As well as Alexander's granite features, 240 ft high and 170 ft wide, a",
  "planned", 
  "---------------------", 
  "So is this mountain limestone or granite?")

So lines 4 and 8 are empty, and I want grep to return those indices.

Answer 1

In R you need to escape the backslash so it should be ^\\\\s*$ . You can also use ^[[:space:]]*$ .

grepl("^[[:space:]]*$", c("", "  ", "\t"))
# [1] TRUE TRUE TRUE
grep("^[[:space:]]*$", string)
# [1] 4 8