如何在正则表达式中跟随某些单位的数字？

Question

Extract the numbers (return only integers) that are followed by the units 'ml' or 'lb' in the following text:

myText <- 'At 10am we had 40ml of water, 5 lb of meat, 3lb of salt and 
20 apples.'

Could anyone help?

Answer 1

We can use str_extract with regex lookaround

library(stringr)
as.numeric(str_extract_all(myText, "\\d+(?=\\s*(ml|lb))")[[1]])
#[1] 40  5  3