diff --git a/src/args.rs b/src/args.rs
new file mode 100644
index 0000000..3199edf
--- /dev/null
+++ b/src/args.rs
@@ -0,0 +1,290 @@
+//! Arguments and Cli-parsing
+use super::*;
+use std::{
+    str,
+    error, fmt,
+    borrow::{
+	Borrow, Cow, ToOwned,
+    },
+    path::{
+	Path, PathBuf,
+    },
+    //collections::BTreeSet as Set,
+};
+use clap::{
+    Parser,
+    Args,
+    Subcommand,
+    ValueEnum,
+};
+
+/// A value that may be provided, or may be deferred to be provided by `stdin` (/ written to `stdout`.)
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
+pub enum MaybeValue<T = String>
+{
+    Stdio,
+    Value(T),
+}
+
+impl<T> MaybeValue<T>
+{
+    pub const STDIO_SYMBOL: &'static str = "-";
+    
+    #[inline] 
+    pub const fn is_stdio(&self) -> bool
+    {
+	match self {
+	    Self::Stdio => true,
+	    _ => false,
+	}
+    }
+    #[inline] 
+    pub const fn value(&self) -> Option<&T>
+    {
+	match self {
+	    Self::Value(v) => Some(&v),
+	    _ => None
+	}
+    }
+    #[inline(always)] 
+    pub const fn has_value(&self) -> bool
+    {
+	self.value().is_some()
+    }
+
+    /// Convert the value type to `U` (if there is one.)
+    ///
+    /// e.g. to convert `let _: MaybeValue<PathBuf> = MaybeString::map_into();`
+    #[inline] 
+    pub fn map_into<U: From<T>>(self) -> MaybeValue<U>
+    {
+	match self {
+	    Self::Value(v) => MaybeValue::Value(v.into()),
+	    Self::Stdio => MaybeValue::Stdio,
+	}
+    }
+
+    /// Consume into the `Value(T)` if possible, if not, return `Err(Self)`.
+    #[inline]
+    #[must_use]
+    pub fn try_into_value(self) -> Result<T, Self>
+    {
+	match self {
+	    x @ Self::Stdio => Err(x),
+	    Self::Value(v) => Ok(v),
+	}
+    }
+}
+
+impl<T: AsRef<str>> AsRef<str> for MaybeValue<T>
+{
+    #[inline] 
+    fn as_ref(&self) -> &str
+    {
+	match self {
+	    Self::Stdio => Self::STDIO_SYMBOL,
+	    Self::Value(v) => v.as_ref(),
+	}
+    }
+}
+
+impl<T: Borrow<str>> Borrow<str> for MaybeValue<T>
+{
+    #[inline] 
+    fn borrow(&self) -> &str
+    {
+	match self {
+	    Self::Stdio => Self::STDIO_SYMBOL,
+	    Self::Value(v) => v.borrow(),
+	}
+    }
+}
+
+
+impl<T> Default for MaybeValue<T>
+{
+    #[inline]
+    fn default() -> Self
+    {
+	Self::Stdio
+    }
+}
+
+impl<T: Into<String>> MaybeValue<T>
+{
+    #[inline] 
+    pub fn into_string(self) -> Cow<'static, str>
+    {
+	match self {
+	    Self::Value(v) => Cow::Owned(v.into()),
+	    Self::Stdio => Cow::Borrowed(Self::STDIO_SYMBOL),
+	}
+    }
+}
+
+impl<T: Into<PathBuf>> MaybeValue<T>
+{
+    #[inline] 
+    pub fn into_path(self) -> Cow<'static, Path>
+    {
+	match self {
+	    Self::Value(v) => Cow::Owned(v.into()),
+	    Self::Stdio => Cow::Borrowed(Path::new(Self::STDIO_SYMBOL)),
+	}
+    }
+}
+
+impl<T: Into<PathBuf>> From<MaybeValue<T>> for Box<Path>
+{
+    #[inline] 
+    fn from(value: MaybeValue<T>) -> Self {
+	value.into_path().into_owned().into_boxed_path()
+    }
+}
+impl<T: Into<String>> From<MaybeValue<T>> for Box<str>
+{
+    #[inline] 
+    fn from(value: MaybeValue<T>) -> Self {
+	value.into_string().into_owned().into_boxed_str()
+    }
+}
+
+impl<T: Into<String>> From<MaybeValue<T>> for Cow<'static, str>
+{
+    fn from(from: MaybeValue<T>) -> Self
+    {
+	from.into_string()
+    }
+}
+
+
+impl<T: From<String>> From<String> for MaybeValue<T>
+{
+    #[inline] 
+    fn from(from: String) -> Self
+    {
+	match &from[..] {
+	    Self::STDIO_SYMBOL => Self::Stdio,
+	    _ => Self::Value(from.into()),
+	}
+    }
+}
+
+impl<T> str::FromStr for MaybeValue<T>
+where T: str::FromStr {
+    type Err = T::Err;
+    
+    #[inline] 
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+	match s {
+	    Self::STDIO_SYMBOL => Ok(Self::Stdio),
+	    s => T::from_str(s).map(Self::Value)
+	}
+    }
+}
+
+/// User-provied configuration of how the program should behave here
+#[derive(Debug, Args)]
+pub struct Config
+{
+    /// Use the PCRE (JS-like) extended regular expression compiler
+    ///
+    /// # Feature difference 
+    /// By default, the expression syntax does not support things like negative lookahead and other backtrack-requiring regex features.
+    ///
+    /// ## Efficiency
+    /// Note that non-PCRE expressions are more efficient in general, and can also enable parallel processing of strings where there are many (e.g. a long list of lines from `stdin` can be matched against in parallel.)
+    ///
+    /// It is ill-advised to enable PCRE on large inputs unless those features are required.
+    //TODO: Should we have PCRE on by default or not...? I think we should maybe have it on by default if the feature is enabled... But that will mess with input parallelism... XXX: Perhaps we can auto-detect if to use PCRE or not (e.g. try compiling to regex first, then PCRE if that fails?) 
+    #[arg(short, long)]
+    #[cfg(feature="perl")]  //XXX: Do we want this option to be feature-gated? Or should we fail with error `if (! cfg!(feature="perl")) && self.extended)`? I think the latter would make things more easily (since the Regex engine gates PCRE-compilation transparently to the API user [see `crate::re::Regex`], we don't need to gate it this way outside of `re`, if we remove this gate we can just use `cfg!()` everywhere here which makes things **MUCH** cleaner..) It also means the user of a non-PCRE build will at least know why their PCRE flag is failing and that it can be built with the "perl" feature, instead of it being *totally* invisible to the user if the feature is off.
+    extended: bool,
+
+    /// Delimit read input/output strings from/to `stdin`/`stdout` by NUL ('\0') characters instead of newlines.
+    ///
+    /// This only affects the output of each string's match groups, not the groups themselves, those will still be delimited by TAB literals in the output.
+    #[arg(short='0', long)]
+    pub zero: bool, //XXX: Add `--field=`/`--ifs` option, put these in same group. Maybe add `--delimit-groups=` to change the group delimiter from `\t` to user-specified value. 
+}
+
+impl Config
+{
+    /// Whether it is requested to use PCRE regex instead of regular regex.
+    ///
+    /// # Interaction with feature gating of ~actual~ PCRE support via `feature="perl"`
+    /// Note that if the "perl" feature is not enabled, this may still return `true`.
+    /// If the user requests PCRE where it is not available, the caller should return an error/panic to the user telling her that.
+    #[inline] 
+    pub fn use_pcre(&self) -> bool
+    {
+	#![allow(unreachable_code)]
+	#[cfg(feature="perl")] return self.extended; //TODO: See above comment on un-gating `self.extended`
+	false
+    }
+}
+
+/// A string value that may be provided to the CLI, or delegated to `stdio`.
+pub type MaybeString = MaybeValue<Box<str>>;
+/// A path that may represent an `stdio` file-descriptor instead of a named file.
+pub type MaybePath = MaybeValue<Box<Path>>;
+
+/// `rematch` is a simple command-line tool for matching & printing capture groups of an input string(s) against a regular expression.
+///
+/// The input string(s) can be provided in the command-line, or they can be provided as line delimited (by default) stream from `stdin`.
+#[derive(Debug, Parser)]
+#[command(name = env!("CARGO_PKG_NAME"), version, about, long_about)]
+pub struct Cli
+{
+    /// Configuration of the execution
+    #[command(flatten)]
+    pub config: Config,
+    
+    //XXX: Should we make these fields public?
+    /// The input string to use, or `-` to read from stdin.
+    //TODO: Support multiple input strings in non-`stdin` case too. (XXX: How should this be handled...?)
+    string: MaybeString,
+    /// The regular expression to match `string` on.
+    regex: String,
+    /// The regex capture group indecies to print when matches on `string`.
+    #[arg(required= true, trailing_var_arg = true, allow_hyphen_values = false, num_args=1..)]
+    //TODO: Allow ranges & fallible captures, so lines that match group 1 but not 2 will not cause output failure if given `1 2?` but will if given `1 2` (XXX: Is this actually meaningful/possible? Can we do this at all? I'm pretty sure `/(?:(.))?/` still creates an (empty) group? So perhaps, syntax for failing on *empty* group matches...? like, `1! 2` for "group #1 *required*, group #2 is not requested?")
+    groups: Vec<usize>, // TODO: How to dedup (XXX: Do we want to de-dup? Maybe the user wants group `1` twice? I think it's fine (also we need to preserve user ordering of group indecied))
+}
+
+impl Cli {
+    /// Get the input string to match on
+    ///
+    /// If the requested input is `stdin`, `None` is returned.
+    #[inline] 
+    pub fn input_string(&self) -> Option<&str>
+    {
+	self.string.value().map(AsRef::as_ref)
+    }
+
+    /// Get the string to build the regular expression from
+    pub fn regex_string(&self) -> &str
+    {
+	&self.regex[..]
+    }
+
+    /// Get the match group(s) to print in the output
+    #[inline] 
+    pub fn groups(&self) -> &[usize]
+    {
+	&self.groups[..]
+    }
+
+    /// Get the number of match groups requested.
+    #[inline] 
+    pub fn num_groups(&self) -> usize
+    {
+	self.groups.len()
+    }
+}
+
+/// Parse the command-line arguments passed to the program
+pub fn parse_cli() -> Cli
+{
+    clap::Parser::parse()
+}
diff --git a/src/main.rs b/src/main.rs
index 9c2ace6..4fad2dc 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,7 @@
 
 mod re;
 mod text;
+mod args;
 
 use color_eyre::{
     eyre::{
@@ -20,6 +21,10 @@ fn initialise() -> eyre::Result<()>
 fn main() -> eyre::Result<()>
 {
     initialise().wrap_err("Fatal: Failed to install panic handle")?;
+    let cli = args::parse_cli();//.wrap_err("Error parsing command-line arguments")?;
+
+    eprintln!("{:#?}", cli);
+    return Ok(());
 
     let args: Vec<String> = std::env::args().collect();