与 R 中的 grepl() 函数进行模式匹配

Question

I am confused on below result on pattern match using grepl() function -

grepl("[0-9]{2}-[0-9]{2}-[0-9]{2}", "2010-04-09") # TRUE
grepl("[0-9]{4}-[0-9]{2}-[0-9]{2}", "2010-04-09")  #TRUE

Shouldn't I expect the first result to be FALSE ?

Any pointer will be highly appreciated.

Answer 1

The result is correct.

grepl is looking for the pattern of xx-xx-xx, where x is a digit, and that does appear in the first query. If you want to query starting from the beginning of the string, you can use the ^ symbol.

For example, if you were to run grepl("^[0-9]{2}-[0-9]{2}-[0-9]{2}", "2010-04-09") , you'd get FALSE, but grepl("^[0-9]{4}-[0-9]{2}-[0-9]{2}", "2010-04-09") would return TRUE.

PS: On the opposite end, $ indicates the end of the string.