Extracting decimal numbers from a string

Question

I have a string such as "3.1 ml" or "abc 3.1 xywazw"

I'd like to extract "3.1" from this string. I have found many questions on stackoverflow about the extraction of numbers from a character string, but no solution works for the case of decimal numbers.

Answer 1

This approach makes the decimal point and decimal fraction optional and allows multiple numbers to be extracted:

str <- " test 3.1 test 5"
as.numeric(unlist(regmatches(str,
                             gregexpr("[[:digit:]]+\\.*[[:digit:]]*",str))
          )      )
#[1] 3.1 5.0

The concern about negative numbers can be address with optional perl style look-ahead:

 str <- " test -4.5 3.1 test 5"
    as.numeric(unlist(regmatches(str,gregexpr("(?>-)*[[:digit:]]+\\.*[[:digit:]]*",str, perl=TRUE))))

#[1] -4.5  3.1  5.0

Answer 2

Use the stringr library:

x<-"abc 3.1 xywazw"
str_extract(x, "\\d+\\.*\\d*")
[1] "3.1"

Answer 3

Regular expression for floating point number from http://www.regular-expressions.info/floatingpoint.html with minor adjustment to work in R.

s <- "1e-6 dkel"
regmatches(s,gregexpr("[-+]?[0-9]*\\.?[0-9]+([eE][-+]?[0-9]+)?",s)) 
> [[1]]
> [1] "1e-6"

Answer 4

You can use regular expressions :

> str <- " test 3.1 test"
> as.numeric(regmatches(str,regexpr("[[:digit:]]+\\.[[:digit:]]+",str)))
[1] 3.1

regexprreturns the start position and length of the matched string. regmatchesreturns the matches. You can then convert it to a number.