Regex scan fails - regex

Question

I am trying to parse all money from a string. For example, I want to extract:
['$250,000', '$3.90', '$250,000', '$500,000']
from:
'Up to $250,000………………………………… $3.90 Over $250,000 to $500,000'
The regex:
\$\ ?(\d+\,)*\d+(\.\d*)?
seems to match all money expressions as in this link. However, when I try to scan on Ruby, it fails to give me the desired result.
s # => "Up to $250,000 $3.90 Over $250,000 to $500,000, add$3.70 Over $500,000 to $1,000,000, add..$3.40 Over $1,000,000 to $2,000,000, add...........$2.25\nOver $2,000,000 add ..$2.00"
r # => /\$\ ?(\d+\,)*\d+\.?\d*/
s.scan(r)
# => [["250,"], [nil], ["250,"], ["500,"], [nil], ["500,"], ["000,"], [nil], ["000,"], ["000,"], [nil], ["000,"], [nil]]
From String#scan docs, it looks like this is because of the group. How can I parse all the money in the string?

Answer 1

Let's look at your regular expression, which I'll write in free-spacing mode so I can document it:
r = /
\$ # match a dollar sign
\ ? # optionally match a space (has no effect)
( # begin capture group 1
\d+ # match one or more digits
, # match a comma (need not be escaped)
)* # end capture group 1 and execute it >= 0 times
\d+ # match one or more digits
\.? # optionally match a period
\d* # match zero or more digits
/x # free-spacing regex definition mode
In non-free-spacing mode this would be written as follows.
r = /\$ ?(\d+,)*\d+\.?\d*/
When a regex is defined in free-spacing mode all spaces are stripped out before the regex is evaluated, which is why I had to escape the space. That's not necessary when the regex is not defined in free-spacing mode.
It is nowhere needed to match a space after the dollars sign, so \ ? should be removed. Suppose now we have
r = /\$\d+\.?\d*/
"$2.31 cat $44. dog $33.607".scan r
#=> ["$2.31", "$44.", "$33.607"]
That works, but it is questionable whether you want to match values that do not have exactly two digits after the decimal point.
Now write
r = /\$(\d+,)*\d+\.?\d*/
"$2.31 cat $44. dog $33.607".scan r
#=> [[nil], [nil], [nil]]
To see why this result was obtained examine the doc for String#scan, specifically the last sentence of the first paragraph: " If the pattern contains groups, each individual result is itself an array containing one entry per group.".
We can avoid that problem by changing the capture group to a non-capture group:
r = /\$(?:\d+,)*\d+\.?\d*/
"$2.31 cat $44. dog $33.607".scan r
#=> ["$2.31", "$44.", "$33.607"]
Now consider this:
"$2,241.31 cat $1,2345. dog $33.607".scan r
#=> ["$2,241.31", "$1,2345.", "$33.607"]
which is still not quite right. Try the following.
r = /
\$ # match a dollar sign
\d{1,3} # match one to three digits
(?:,\d{3}) # match ',' then 3 digits in a nc group
* # execute the above nc group >=0 times
(?:\.\d{2}) # match '.' then 2 digits in a nc group
? # optionally match the above nc group
(?![\d,.]) # no following digit, ',' or '.'
/x # free-spacing regex definition mode
"$2,241.31 $2 $1,234 $3,6152 $33.607 $146.27".scan r
#=> ["$2,241.31", "$2", "$1,234", "$146.27"]
(?![\d,.]) is a negative lookahead.
In normal mode this regular expression is written as follows.
r = /\$\d{1,3}(?:,\d{3})*(?:\.\d{2})?(?![\d,.])/
The following erroneous result would obtain without the negative lookahead at the end of the regex.
r = /\$\d{1,3}(?:,\d{3})*(?:\.\d{2})?/
"$2,241.31 $2 $1,234 $3,6152 $33.607 $146.27".scan r
#=> ["$2,241.31", "$2", "$1,234", "$3,615", "$33.60",
# "$146.27"]

Answer 2

[3] pry(main)> str = <<EOF
[3] pry(main)* Up to $250,000………………………………… $3.90 Over $250,000 to $500,000, add………………$3.70 Over $500,000 to $1,000,000, add……………..$3.40 Over $1,000,000 to $2,000,000, add……...........$2.25
[3] pry(main)* Over $2,000,000 add …..………………………$2.00
[3] pry(main)* EOF
=> "Up to $250,000………………………………… $3.90 Over $250,000 to $500,000, add………………$3.70 Over $500,000 to $1,000,000, add……………..$3.40 Over $1,000,000 to $2,000,000, add……...........$2.25\nOver $2,000,000 add …..………………………$2.00\n"
[4] pry(main)> str.scan /\$\d+(?:[,.]\d+)*/
=> ["$250,000", "$3.90", "$250,000", "$500,000", "$3.70", "$500,000", "$1,000,000", "$3.40", "$1,000,000", "$2,000,000", "$2.25", "$2,000,000", "$2.00"]
[5] pry(main)>