Parse formatted number string to number

src/FormatNumber.elm

@@ -1,4 +1,4 @@
-module FormatNumber exposing (format)
+module FormatNumber exposing (format, parse)
 
 {-| This simple package formats `Float` numbers as pretty strings. It is
 flexible enough to deal with different number of decimals, different thousand
@@ -22,7 +22,7 @@ Just convert them to `Float` before passing them to `format`:
 -}
 
 import FormatNumber.Locales as Locales
-import FormatNumber.Parser exposing (parse)
+import FormatNumber.Parser as Parser
 import FormatNumber.Stringfy exposing (stringfy)
 
 
@@ -139,5 +139,23 @@ import FormatNumber.Stringfy exposing (stringfy)
 format : Locales.Locale -> Float -> String
 format locale number_ =
     number_
-        |> parse locale
+        |> Parser.parse locale
         |> stringfy locale
+
+{-| Parses a pretty string into `Maybe Float`:
+
+    import FormatNumber.Locales exposing (Decimals(..), Locale, base, frenchLocale, spanishLocale, usLocale)
+
+    parse { base | thousandSeparator = "," } "100,000.345"
+    --> Just 100000.345
+
+    parse { base | thousandSeparator = "," } "-100,000"
+    --> Just -100000
+
+    parse { base | thousandSeparator = ",", negativePrefix = "(", negativeSuffix = ")" } "(100,000.546)"
+    --> Just -100000.546
+
+-}
+parse : Locales.Locale -> String -> Maybe Float
+parse locale str = 
+    Parser.parseString locale str

src/FormatNumber/Parser.elm

@@ -4,6 +4,7 @@ module FormatNumber.Parser exposing
     , addZerosToFit
     , classify
     , parse
+    , parseString
     , removeZeros
     , splitInParts
     , splitThousands
@@ -13,6 +14,7 @@ import Char
 import FormatNumber.Locales exposing (Decimals(..), Locale)
 import Round
 import String
+import Regex
 
 
 {-| `Category` is a helper type and constructor to classify numbers in positive
@@ -378,3 +380,40 @@ parse locale original =
                 | prefix = locale.zeroPrefix
                 , suffix = locale.zeroSuffix
             }
+
+{-| Given a `Locale` parses a `String` into a `Maybe Float`:
+-}
+parseString : Locale -> String -> Maybe Float
+parseString locale value = 
+    let
+        isNegative : Bool
+        isNegative = 
+            if String.left 1 value == locale.negativePrefix then
+                True
+            else
+                False
+        chrs : Regex.Regex
+        chrs = 
+            ["[", locale.negativePrefix, locale.negativeSuffix, locale.thousandSeparator, "]"]
+                |> String.concat
+                |> Regex.fromString
+                |> Maybe.withDefault Regex.never
+        splitValue : String -> List String
+        splitValue nbr = 
+            nbr
+                |> Regex.replace chrs (\_ -> "")
+                |> String.split locale.decimalSeparator
+        sumNumber : Float -> Float -> Float -> Float
+        sumNumber i d pow =
+            if isNegative then
+                -1 * (i + d/(10^pow))
+            else
+                i + d/(10^pow)
+    in
+        case splitValue value of
+            integer :: decimal :: [] ->
+                Maybe.map3 (sumNumber) (String.toFloat integer) (String.toFloat decimal) (Just (toFloat (String.length decimal)))
+            integer :: [] ->
+                Maybe.map (\n -> if isNegative then -1 * n else n) (String.toFloat integer)
+            _ -> 
+                Nothing