Extracting positive/negative floating-point numbers from a string

Question

I am trying to extract numbers out of some text. Currently I am using the following:

echo "2.5 test. test -50.8" | tr '\n' ' ' | sed -e 's/[^0-9.]/ /g' -e 's/^ *//g' -e 's/ *$//g' | tr -s ' '

This would give me 2.5, "." and 50.8. How should I modify the first sed so it would detect float numbers, both positive and negative?

Answer 1

grep works well for this:

$ echo "2.5 test. test -50.8" | grep -Eo '[+-]?[0-9]+([.][0-9]+)?'
2.5
-50.8

How it works

• -E

  Use extended regex.

• -o

  Return only the matches, not the context

• [+-]?[0-9]+([.][0-9]+)?+

  Match numbers which are identified as:

  • [+-]?

    An optional leading sign

  • [0-9]+

    One or more numbers

  • ([.][0-9]+)?

    An optional period followed by one or more numbers.

Getting the output on one line

$ echo "2.5 test. test -50.8" | grep -Eo '[+-]?[0-9]+([.][0-9]+)?' | tr '\n' ' '; echo ""
2.5 -50.8

Answer 2

A grep solution:

$ echo "2.5 test. test -50.8" | tr ' ' '\n' | grep -E '^[+-]?[0-9]*\.?([0-9]+)$'
2.5
-50.8

• The tr just converts the line into multiple lines by replacing the spaces with newlines.

• The grep command looks for strings that starts with an optional + or -, possibly followed by some digits and an optional decimal point. Then we require some digits at the end.

This will let through things like 00000123.91288000, which just looks strange. Is this a number we want to filter out or not? It's technically a floating point number, just oddly formatted.

EDIT: To properly check for numbers, do not write your own regular expression! Use a library routine from somewhere reliable.

In my case, I would use Perl's Scalar::Util package, which has a convenient looks_like_number() subroutine:

$ echo "2.5 test. test -50.8" | tr ' ' '\n' | perl -MScalar::Util -ne 'Scalar::Util::looks_like_number($_) && print'
2.5
-50.8

This has the added benefit of finding numbers on other forms, such as 1e3.