# strscans

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This module contains a scanf macro that can be used for extracting substrings from an input string. This is often easier than regular expressions. Some examples as an appetizer:

    # check if input string matches a triple of integers:
    const input = "(1,2,4)"
    var x, y, z: int
    if scanf(input, "($i,$i,$i)", x, y, z):
      echo "matches and x is ", x, " y is ", y, " z is ", z

    # check if input string matches an ISO date followed by an identifier followed
    # by whitespace and a floating point number:
    var year, month, day: int
    var identifier: string
    var myfloat: float
    if scanf(input, "$i-$i-$i $w$s$f", year, month, day, identifier, myfloat):
      echo "yes, we have a match!"

As can be seen from the examples, strings are matched verbatim except for substrings starting with `$`. These constructions are available:

`$b` | Matches a binary integer. This uses `parseutils.parseBin`.
`$o` | Matches an octal integer. This uses `parseutils.parseOct`.
`$i` | Matches a decimal integer. This uses `parseutils.parseInt`.
`$h` | Matches a hex integer. This uses `parseutils.parseHex`.
`$f` | Matches a floating pointer number. Uses `parseFloat`.
`$w` | Matches an ASCII identifier: `[A-Za-z_][A-Za-z_0-9]*`.
`$s` | Skips optional whitespace.
`$$` | Matches a single dollar sign.
`$.` | Matches if the end of the input string has been reached.
`$*` | Matches until the token following the `$*` was found. The match is allowed to be of 0 length.
`$+` | Matches until the token following the `$+` was found. The match must consist of at least one char.
`${foo}` | User defined matcher. Uses the proc `foo` to perform the match. See below for more details.
`$[foo]` | Call user defined proc `foo` to skip some optional parts in the input string. See below for more details.

Even though `$*` and `$+` look similar to the regular expressions `.*` and `.+` they work quite differently, there is no non-deterministic state machine involved and the matches are non-greedy. `[$*]` matches `[xyz]` via `parseutils.parseUntil`.

Furthermore no backtracking is performed, if parsing fails after a value has already been bound to a matched subexpression this value is not restored to its original value. This rarely causes problems in practice and if it does for you, it's easy enough to bind to a temporary variable first.

# Startswith vs full match

`scanf` returns true if the input string starts with the specified pattern. If instead it should only return true if there is also nothing left in the input, append `$.` to your pattern.

# User definable matchers

One very nice advantage over regular expressions is that `scanf` is extensible with ordinary Nim procs. The proc is either enclosed in `${}` or in `$[]`. `${}` matches and binds the result to a variable (that was passed to the `scanf` macro) while `$[]` merely optional tokens.

In this example, we define a helper proc `someSep` that skips some separators which we then use in our scanf pattern to help us in the matching process:

    proc someSep(input: string; start: int; seps: set[char] = {':','-','.'}): int =
      # Note: The parameters and return value must match to what ``scanf`` requires
      result = 0
      while start+result < input.len and input[start+result] in seps: inc result

    if scanf(input, "$w$[someSep]$w", key, value):
      ...

It also possible to pass arguments to a user definable matcher:

    proc ndigits(input: string; intVal: var int; start: int; n: int): int =
      # matches exactly ``n`` digits. Matchers need to return 0 if nothing
      # matched or otherwise the number of processed chars.
      var x = 0
      var i = 0
      while i < n and i+start < input.len and input[i+start] in {'0'..'9'}:
        x = x * 10 + input[i+start].ord - '0'.ord
        inc i
      # only overwrite if we had a match
      if i == n:
        result = n
        intVal = x

    # match an ISO date extracting year, month, day at the same time.
    # Also ensure the input ends after the ISO date:
    var year, month, day: int
    if scanf("2013-01-03", "${ndigits(4)}-${ndigits(2)}-${ndigits(2)}$.", year, month, day):
      ...

# The scanp macro

This module also implements a `scanp` macro, which syntax somewhat resembles an EBNF or PEG grammar, except that it uses Nim's expression syntax and so has to use prefix instead of postfix operators.

`(E)` | Grouping
`*E` | Zero or more
`+E` | One or more
`?E` | Zero or One
`E{n,m}` | From `n` up to `m` times `E`
`~E` | Not predicate
`a ^* b` | Shortcut for `?(a *(b a))`. Usually used for separators.
`a ^+ b` | Shortcut for `?(a +(b a))`. Usually used for separators.
`'a'` | Matches a single character
`{'a'..'b'}` | Matches a character set
`"s"` | Matches a string
`E -> a` | Bind matching to some action
`$_` | Access the currently matched character

Note that unordered or ordered choice operators (`/`, `|`) are not implemented.

Simple example that parses the `/etc/passwd` file line by line:

    const
      etc_passwd = """root:x:0:0:root:/root:/bin/bash
    daemon:x:1:1:daemon:/usr/sbin:/bin/sh
    bin:x:2:2:bin:/bin:/bin/sh
    sys:x:3:3:sys:/dev:/bin/sh
    nobody:x:65534:65534:nobody:/nonexistent:/bin/sh
    messagebus:x:103:107::/var/run/dbus:/bin/false
    """

    proc parsePasswd(content: string): seq[string] =
      result = @[]
      var idx = 0
      while true:
        var entry = ""
        if scanp(content, idx, +(~{'\L', '\0'} -> entry.add($_)), '\L'):
          result.add entry
        else:
          break

The `scanp` maps the grammar code into Nim code that performs the parsing. The parsing is performed with the help of 3 helper templates that that can be implemented for a custom type.

These templates need to be named `atom` and `nxt`. `atom` should be overloaded to handle both single characters and sets of character.

    import streams

    template atom(input: Stream; idx: int; c: char): bool =
      ## Used in scanp for the matching of atoms (usually chars).
      peekChar(input) == c

    template atom(input: Stream; idx: int; s: set[char]): bool =
      peekChar(input) in s

    template nxt(input: Stream; idx, step: int = 1) =
      inc(idx, step)
      setPosition(input, idx)

    if scanp(content, idx, +( ~{'\L', '\0'} -> entry.add(peekChar($input))), '\L'):
      result.add entry

Calling ordinary Nim procs inside the macro is possible:

    proc digits(s: string; intVal: var int; start: int): int =
      var x = 0
      while result+start < s.len and s[result+start] in {'0'..'9'} and s[result+start] != ':':
        x = x * 10 + s[result+start].ord - '0'.ord
        inc result
      intVal = x

    proc extractUsers(content: string): seq[string] =
      # Extracts the username and home directory
      # of each entry (with UID greater than 1000)
      const
        digits = {'0'..'9'}
      result = @[]
      var idx = 0
      while true:
        var login = ""
        var uid = 0
        var homedir = ""
        if scanp(content, idx, *(~ {':', '\0'}) -> login.add($_), ':', * ~ ':', ':',
                digits($input, uid, $index), ':', *`digits`, ':', * ~ ':', ':',
                *('/', * ~{':', '/'}) -> homedir.add($_), ':', *('/', * ~{'\L', '/'}), '\L'):
          if uid >= 1000:
            result.add login & " " & homedir
        else:
          break

When used for matching, keep in mind that likewise scanf, no backtracking is performed.

    proc skipUntil(s: string; until: string; unless = '\0'; start: int): int =
      # Skips all characters until the string `until` is found. Returns 0
      # if the char `unless` is found first or the end is reached.
      var i = start
      var u = 0
      while true:
        if i >= s.len or s[i] == unless:
          return 0
        elif s[i] == until[0]:
          u = 1
          while i+u < s.len and u < until.len and s[i+u] == until[u]:
            inc u
          if u >= until.len: break
        inc(i)
      result = i+u-start

    iterator collectLinks(s: string): string =
      const quote = {'\'', '"'}
      var idx, old = 0
      var res = ""
      while idx < s.len:
        old = idx
        if scanp(s, idx, "<a", skipUntil($input, "href=", '>', $index),
                `quote`, *( ~`quote`) -> res.add($_)):
          yield res
          res = ""
        idx = old + 1

    for r in collectLinks(body):
      echo r

In this example both macros are combined seamlessly in order to maximise efficiency and perform different checks.

    iterator parseIps*(soup: string): string =
      ## ipv4 only!
      const digits = {'0'..'9'}
      var a, b, c, d: int
      var buf = ""
      var idx = 0
      while idx < soup.len:
        if scanp(soup, idx, (`digits`{1,3}, '.', `digits`{1,3}, '.',
                 `digits`{1,3}, '.', `digits`{1,3}) -> buf.add($_)):
          discard buf.scanf("$i.$i.$i.$i", a, b, c, d)
          if (a >= 0 and a <= 254) and
             (b >= 0 and b <= 254) and
             (c >= 0 and c <= 254) and
             (d >= 0 and d <= 254):
            yield buf
        buf.setLen(0) # need to clear `buf` each time, cause it might contain garbage
        idx.inc

# Imports

macros, parseutils

# Macros

    macro scanf(input: string; pattern: static[string]; results: varargs[typed]): bool

  See top level documentation of this module about how `scanf` works. Source Edit

    macro scanp(input, idx: typed; pattern: varargs[untyped]): bool

  See top level documentation of this module about how `scanp` works. Source Edit

# Templates

    template atom(input: string; idx: int; c: char): bool

  Used in scanp for the matching of atoms (usually chars). EOF is matched as `'\0'`. Source Edit

    template atom(input: string; idx: int; s: set[char]): bool

  Source Edit

    template hasNxt(input: string; idx: int): bool

  Source Edit

    template success(x: int): bool

  Source Edit

    template nxt(input: string; idx, step: int = 1)

  Source Edit