I've been using the library FsVerbalExpressions to write some functions. I'm having a hard time trying to build a regEx programmatically.
For example, if I have a string "Int. Bus. Mach", I can remove periods and whitespaces and end up with the array
let splitString = [|"Int"; "Bus"; "Mach"|]
What I'd like to do is build a regular expression from splitString so that its result is:
let hardCoded =
VerbEx()
|> startOfLine
|> then' "Int"
|> anything
|> whiteSpace
|> then' "Bus"
|> anything
|> whiteSpace
|> then' "Mach"
hardCoded;;
val it : VerbEx =
^(Int)(.*)\s(Bus)(.*)\s(Mach) {MatchTimeout = -00:00:00.0010000;
Regex = ^(Int)(.*)\s(Bus)(.*)\s(Mach);
RegexOptions = None;
RightToLeft = false;}
My problem is that I don't know how to build this programmatically so that, if the original string is "This is a much bigger string", the entire regEx is built from code rather than hard coded. I can create individual regular expressions with
let test =
splitString
|> Array.map (fun thing -> VerbEx()
|> then' thing)
|> Array.toList
but this is a list of VerbEx() rather than a single VerbEx() above.
Does anyone know how I could build a regEx with FsVerbalExpressions programmatically?
Thanks in advance for your help!
Think about it like this: you need to start with some initial value, VerbEx() |> startOfLine, and then apply to it repeating patterns that have the general shape of anything |> whitespace |> then' word.
You can also think about it in inductive terms: you're producing a series of values, where each value is expressed as previousValue |> anything |> whitespace |> then' word - that is, each next value in the series is previous value with some change applied to it. The very last element of such series is your final answer.
Such operation - producing a series of values, where each value is expressed as a modification of the previous one, - is traditionally called fold. And sure enough, F# has standard library functions for performing this operation:
let applyChange previousValue word =
previousValue |> anything |> whitespace |> then' word
let initialValue = VerbEx() |> startOfLine
let finalAnswer = splitString |> Array.fold applyChange initialValue
Or you can roll that all together:
let finalAnswer =
splitString
|> Array.fold
(fun previousValue word -> previousValue |> anything |> whitespace |> then' word)
(VerbEx() |> startOfLine)
Related
I am trying to work out whether there is a particularly neat or efficient way of truncating a string after the final occurrence of a specific element. For my purposes, it is a monomorphized string list and the string I am looking for the final (highest index) occurrence of is known at compile-time, since I am only using it in one case.
The motivation for this is to find the nearest ancestor in a Unix directory system of the CWD whose name in its parent is a particular folder name. I.E., if I wanted to find the nearest ancestor called bin and I was running the executable from a CWD of /home/anon/bin/projects/sample/src/bin/foo/, then I would want to get back /home/anon/bin/projects/sample/src/bin
The current implementation I am using is the following:
let reverse_prune : tgt:string -> string -> string =
let rec drop_until x ys =
match ys with
| [] -> []
| y :: _ when x = y -> ys
| _ :: yt -> drop_until x yt
in
fun ~tgt path ->
String.split_on_char '/' path
|> List.rev |> drop_until tgt |> List.rev |> String.concat "/"
It isn't a particularly common or expensive code-path so there isn't actually a real need to optimize, but since I am still trying to learn practical OCaml techniques, I wanted to know if there was a cleaner way of doing this.
I also realize that it may technically be possible to avoid the string-splitting altogether and just operate on the raw CWD string without splitting it. I am, of course, welcome to such suggestions as well, but I am specifically curious if there is something that would replace the List.rev |> drop_until tgt |> List.rev snippet, rather than solve the overall problem in a different way.
I don't think this has anything to do with OCaml actually since I'd say the easiest way to do this is by using a regular expression:
let reverse_prune tgt path =
let re =
Str.regexp (Format.sprintf {|^[/a-zA-Z_-]*/%s\([/a-zA-Z_-]*\)$|} tgt)
in
Str.replace_first re {|\1|} path
let () =
reverse_prune "bin" "/home/anon/bin/projects/sample/src/bin/foo/"
|> Format.printf "%s#."
Is there a reason you want to reimplement regular expression searching in a string? If no, just use a solution like mine, I'd say.
If you want the part that comes before just change the group:
let reverse_prune tgt path =
let re =
Str.regexp (Format.sprintf {|^\([/a-zA-Z_-]*/\)%s[/a-zA-Z_-]*$|} tgt)
in
Str.replace_first re {|\1|} path
As mentioned above, the goal is to remove square brackets and commas.
My current solution is the following:
Given:
"[40.45694301152436, -3.6907402812214514]"
|> String.replace("[", "")
|> String.replace(",", "")
|> String.replace("]", "")
|> String.split(" ")
|> Enum.map(fn x -> String.to_float(x) end)
Output:
[40.45694301152436, -3.6907402812214514]
I know this can be compacted much more, but I've been looking at examples all day and all failed to do the job above.
Instead of a string, you can pass a regex to String.replace. In Elixir you can build a regex with ~r sigil.
"[40.45694301152436, -3.6907402812214514]"
|> String.replace(~r'[\[\],]', "")
|> String.split()
|> Enum.map(&String.to_float/1)
While #fhdhsni has given you a great answer if your concern is readability I'd suggest abstracting the whole thing to a separate function like so:
defmodule T do
def parsefloats(stringtobeparsed) do
stringtobeparsed
|> String.replace("[", "")
|> String.replace(",", "")
|> String.replace("]", "")
|> String.split(" ")
|> Enum.map(fn x -> String.to_float(x) end)
end
end
Then you call it like so:
[x,y] = T.parsefloats("[40.45694301152436, -3.6907402812214514]")
# [40.45694301152436, -3.6907402812214514]
iex(3)> x
# 40.45694301152436
iex(4)> y
# -3.6907402812214514
Not any better in terms of more compact code but more readable I think.
Here's another option, using String.slice:
"[40.45694301152436, -3.6907402812214514]"
|> String.slice(1..-2)
|> String.split(~r/,\s+/)
|> Enum.map(&String.to_float/1)
Cheers!
Apart from the string replace solutions, you can take a look at Code.eval_string as well.
This way the string will get parsed and you'll get back the list you're looking for;
{list, _} = Code.eval_string "[40.45694301152436, -3.6907402812214514]"
# {[40.45694301152436, -3.6907402812214514], []}
I think the comment on your question about using a JSON parser is best, followed by fhdhsni's simple answer. But here's a method that extracts the numbers, rather than replacing the brackets:
str = "[40.45694301152436, -3.6907402812214514]"
regex = ~r/([\d\.-]+), ([\d\.+-]+)/
Regex.run(regex, str, capture: :all_but_first) |> Enum.map(&String.to_float/1)
Output:
[40.45694301152436, -3.6907402812214514]
I cannot find a way to get a list from values given by the user. I already did
# read_line() |> Str.split (Str.regexp " +") |> List.map int_of_string;;
but don't know how to do the same for chars.
just remember that a string is an array of char:
# let s = "ab";;
# s.[0];;
- : char = 'a'
So, if you have to return a char for any of the elements of your list :
# read_line() |> Str.split (Str.regexp " +") |> List.map (fun x -> x.[0])
From the context of your previous questions, you seem to actually need a way to parse text that contains a mix of integers and characters that describe the transitions for a state machine. I'd suggest that you may be best served with the Scanf module for this. If you need something much more complex, though, this may require a handwritten scanner (you can use Str.string_match for that or – if you're willing to dive that deep – use the ocamllex scanner generator).
A simple example of reading a line from a channel with Scanf would be:
let read_transition input =
try
let line = input_line input in
Scanf.sscanf line "%d %c %d"
(fun x ch y -> Some (x, ch, y))
with End_of_file -> None
Note that we're reading the line with input_line rather than using Scanf.scanf on the input channel directly. The reason is that Scanf.scanf may need up to a character worth of lookahead and thus, if you mix it with other ways of reading the channel, characters may get skipped. By using input_line and then Scanf.sscanf (rather than Scanf.scanf) we avoid that corner case.
Note also that depending on the syntax of the input, you may have to adjust the scanf pattern accordingly.
I am new to programming and F# is my first .NET language.
As a beginner's project, I would like to write an application asking the user to enter a regex pattern and then flagging any errors.
I have looked through the Regex API on MSDN but there doesn't seem to be any methods that would automatically detect any errors in regex patterns. Will more experienced programmers kindly share with me how they would go about accomplishing this?
Thank you in advance for your help.
If you need to check if a regex compiles or not, simply use try-with block. If you need to check if a regex pattern matches your input string, use IsMatch() or .Success. That is quite enough.
An example with code taken from another SO post, but with an error in regex pattern where I replaced (http:\/\/\S+) with (http:\/\/\S+:
try
let testString = "http://www.bob.com http://www.b.com http://www.bob.com http://www.bill.com"
let matches input =
Regex.Matches(input, "(http:\/\/\S+")
|> Seq.cast<Match>
|> Seq.groupBy (fun m -> m.Value)
|> Seq.map (fun (value, groups) -> value, (groups |> Seq.length))
with
| :? System.Exception as ex -> printfn "Exception! %s " (ex.Message); None
More on F# exception raising can be found here or here.
I found this useful article on using Active Patterns with Regular Expressions:
http://www.markhneedham.com/blog/2009/05/10/f-regular-expressionsactive-patterns/
The original code snippet used in the article was this:
open System.Text.RegularExpressions
let (|Match|_|) pattern input =
let m = Regex.Match(input, pattern) in
if m.Success then Some (List.tl [ for g in m.Groups -> g.Value ]) else None
let ContainsUrl value =
match value with
| Match "(http:\/\/\S+)" result -> Some(result.Head)
| _ -> None
Which would let you know if at least one url was found and what that url was (if I understood the snippet correctly)
Then in the comment section Joel suggested this modification:
Alternative, since a given group may
or may not be a successful match:
List.tail [ for g in m.Groups -> if g.Success then Some g.Value else None ]
Or maybe you give labels to your
groups and you want to access them by
name:
(re.GetGroupNames()
|> Seq.map (fun n -> (n, m.Groups.[n]))
|> Seq.filter (fun (n, g) -> g.Success)
|> Seq.map (fun (n, g) -> (n, g.Value))
|> Map.ofSeq)
After trying to combine all of this I came up with the following code:
let testString = "http://www.bob.com http://www.b.com http://www.bob.com http://www.bill.com"
let (|Match|_|) pattern input =
let re = new Regex(pattern)
let m = re.Match(input) in
if m.Success then Some ((re.GetGroupNames()
|> Seq.map (fun n -> (n, m.Groups.[n]))
|> Seq.filter (fun (n, g) -> g.Success)
|> Seq.map (fun (n, g) -> (n, g.Value))
|> Map.ofSeq)) else None
let GroupMatches stringToSearch =
match stringToSearch with
| Match "(http:\/\/\S+)" result -> printfn "%A" result
| _ -> ()
GroupMatches testString;;
When I run my code in an interactive session this is what is output:
map [("0", "http://www.bob.com"); ("1", "http://www.bob.com")]
The result I am trying to achieve would look something like this:
map [("http://www.bob.com", 2); ("http://www.b.com", 1); ("http://www.bill.com", 1);]
Basically a mapping of each unique match found followed by the count of the number of times that specific matching string was found in the text.
If you think I'm going down the wrong path here please feel free to suggest a completely different approach. I'm somewhat new to both Active Patterns and Regular Expressions so I have no idea where to even begin in trying to fix this.
I also came up with this which is basically what I would do in C# translated to F#.
let testString = "http://www.bob.com http://www.b.com http://www.bob.com http://www.bill.com"
let matches =
let matchDictionary = new Dictionary<string,int>()
for mtch in (Regex.Matches(testString, "(http:\/\/\S+)")) do
for m in mtch.Captures do
if(matchDictionary.ContainsKey(m.Value)) then
matchDictionary.Item(m.Value) <- matchDictionary.Item(m.Value) + 1
else
matchDictionary.Add(m.Value, 1)
matchDictionary
Which returns this when run:
val matches : Dictionary = dict [("http://www.bob.com", 2); ("http://www.b.com", 1); ("http://www.bill.com", 1)]
This is basically the result I am looking for, but I'm trying to learn the functional way to do this, and I think that should include active patterns. Feel free to try to "functionalize" this if it makes more sense than my first attempt.
Thanks in advance,
Bob
Interesting stuff, I think everything you are exploring here is valid. (Partial) active patterns for regular expression matching work very well indeed. Especially when you have a string which you want to match against multiple alternative cases. The only thing I'd suggest with the more complex regex active patterns is that you give them more descriptive names, possibly building up a collection of different regex active patterns with differing purposes.
As for your C# to F# example, you can have functional solution just fine without active patterns, e.g.
let testString = "http://www.bob.com http://www.b.com http://www.bob.com http://www.bill.com"
let matches input =
Regex.Matches(input, "(http:\/\/\S+)")
|> Seq.cast<Match>
|> Seq.groupBy (fun m -> m.Value)
|> Seq.map (fun (value, groups) -> value, (groups |> Seq.length))
//FSI output:
> matches testString;;
val it : seq<string * int> =
seq
[("http://www.bob.com", 2); ("http://www.b.com", 1);
("http://www.bill.com", 1)]
Update
The reason why this particular example works fine without active patterns is because 1) you are only testing one pattern, 2) you are dynamically processing the matches.
For a real world example of active patterns, let's consider a case where 1) we are testing multiple regexes, 2) we are testing for one regex match with multiple groups. For these scenarios, I use the following two active patterns, which are a bit more general than the first Match active pattern you showed (I do not discard first group in the match, and I return a list of the Group objects, not just their values -- one uses the compiled regex option for static regex patterns, one uses the interpreted regex option for dynamic regex patterns). Because the .NET regex API is so feature filled, what you return from your active pattern is really up to what you find useful. But returning a list of something is good, because then you can pattern match on that list.
let (|InterpretedMatch|_|) pattern input =
if input = null then None
else
let m = Regex.Match(input, pattern)
if m.Success then Some [for x in m.Groups -> x]
else None
///Match the pattern using a cached compiled Regex
let (|CompiledMatch|_|) pattern input =
if input = null then None
else
let m = Regex.Match(input, pattern, RegexOptions.Compiled)
if m.Success then Some [for x in m.Groups -> x]
else None
Notice also how these active patterns consider null a non-match, instead of throwing an exception.
OK, so let's say we want to parse names. We have the following requirements:
Must have first and last name
May have middle name
First, optional middle, and last name are separated by a single blank space in that order
Each part of the name may consist of any combination of at least one or more letters or numbers
Input may be malformed
First we'll define the following record:
type Name = {First:string; Middle:option<string>; Last:string}
Then we can use our regex active pattern quite effectively in a function for parsing a name:
let parseName name =
match name with
| CompiledMatch #"^(\w+) (\w+) (\w+)$" [_; first; middle; last] ->
Some({First=first.Value; Middle=Some(middle.Value); Last=last.Value})
| CompiledMatch #"^(\w+) (\w+)$" [_; first; last] ->
Some({First=first.Value; Middle=None; Last=last.Value})
| _ ->
None
Notice one of the key advantages we gain here, which is the case with pattern matching in general, is that we are able to simultaneously test that an input matches the regex pattern, and decompose the returned list of groups if it does.