R从字符串末尾提取第一个模式

Question

I want to extract sizes from strings, which can be:

a <- c("xxxxxxx 2.5 oz (23488)",
        "xxxxx /1.36oz",
        "xxxxx/7 days /20 ml")

Result I want: 2.5 oz /1.36oz /20 ml

Because strings varies, so I want to extract patterns backward. That is, I want to extract the first appearance of \\\\/*(\\\\d+\\\\.*\\\\d*)\\\\s*[[:alpha:]]+ from the end of the string. It will avoid R from taking 23488 from the first string and /7 days from the third string.

Anyone knows how I can achieve this? Thanks!

Answer 1

You may use

> a <- c("xxxxxxx 2.5 oz (23488)",
+         "xxxxx /1.36oz",
+         "xxxxx/7 days /20 ml")
> regmatches(a, regexpr("/?\\d+(?:\\.\\d+)?\\s*\\p{L}+(?!.*\\d(?:\\.\\d+)?\\s*\\p{L}+)", a, perl=TRUE))
[1] "2.5 oz"  "/1.36oz" "/20 ml" 

See the regex demo .

Details

• /? - an optional /
• \\\\d+ - 1+ digits
• (?:\\\\.\\\\d+)? - an optional . and 1+ digits sequence
• \\\\s* - 0+ whitespaces
• \\\\p{L}+ - 1+ letters
• (?!.*\\\\d(?:\\\\.\\\\d+)?\\\\s*\\\\p{L}+) - not followed with
  • .* - any 0+ chars, as many as possible
  • \\\\d - a digit
  • (?:\\\\.\\\\d+)? - an optional . and 1+ digits sequence
  • \\\\s* - 0+ whitespaces
  • \\\\p{L}+ - 1+ letters

Answer 2

If you know the name of the units(oz, ml, etc), you could try something like this:

((\\d*|\\d*\\.\\d{0,2})\\s?(ml|oz|etc))

See working example .