R：使用POSIX正则表达式从字符串中提取数据

Question

How to extract only DATABASE_NAME from this string using POSIX-style regular expressions?

st <- "MICROSOFT_SQL_SERVER.DATABASE\INSTANCE.DATABASE_NAME."

First of all, this generates an error

Error: '\I' is an unrecognized escape in character string starting "MICROSOFT_SQL_SERVER.DATABASE\I"

I was thinking something like

sub(".*\\.", st, "")

Answer 1

The first problem is that you need to escape the \\ in your string:

st <- "MICROSOFT_SQL_SERVER.DATABASE\\INSTANCE.DATABASE_NAME."

As for the main problem, this will return the bit you want from the string you gave:

> sub("\\.$", "", sub("[A-Za-z0-9\\._]*\\\\[A-Za-z]*\\.", "", st))
[1] "DATABASE_NAME"

But a simpler solution would be to split on the \\\\. and select the last chunk:

> strsplit(st, "\\.")[[1]][3]
[1] "DATABASE_NAME"

or slightly more automated

> sst <- strsplit(st, "\\.")[[1]]
> tail(sst, 1)
[1] "DATABASE_NAME"

Answer 2

Other answers provided some really good alternative ways of cracking the problem using strsplit or str_split .

However, if you really want to use a regex and gsub , this solution substitutes the first two occurrences of a (string followed by a period) with an empty string.

Note the use of the ? modifier to tell the regex not to be greedy, as well as the {2} modifier to tell it to repeat the expression in brackets two times.

gsub("\\.", "", gsub("(.+?\\.){2}", "", st)) 
[1] "DATABASE_NAME"

Answer 3

An alternative approach is to use str_split in package stringr . The idea is to split st into strings at each period, and then to isolate the third string:

st <- "MICROSOFT_SQL_SERVER.DATABASE\\INSTANCE.DATABASE_NAME."

library(stringr)

str_split(st, "\\.")[[1]][3]

[1] "DATABASE_NAME"