Enum `Look`

pub enum Look

A look-around assertion.

An assertion matches at a position between characters in a haystack. Namely, it does not actually "consume" any input as most parts of a regular expression do. Assertions are a way of stating that some property must be true at a particular point during matching.

For example, (?m)^[a-z]+$ is a pattern that:

Scans the haystack for a position at which (?m:^) is satisfied. That occurs at either the beginning of the haystack, or immediately following a \n character.
Looks for one or more occurrences of [a-z].
Once [a-z]+ has matched as much as it can, an overall match is only reported when [a-z]+ stops just before a \n.

So in this case, abc and \nabc\n match, but \nabc1\n does not.

Assertions are also called "look-around," "look-behind" and "look-ahead." Specifically, some assertions are look-behind (like ^), other assertions are look-ahead (like $) and yet other assertions are both look-ahead and look-behind (like \b).

Assertions in an NFA

An assertion in a thompson::NFA can be thought of as a conditional epsilon transition. That is, a matching engine like the PikeVM only permits moving through conditional epsilon transitions when their condition is satisfied at whatever position the PikeVM is currently at in the haystack.

How assertions are handled in a DFA is trickier, since a DFA does not have epsilon transitions at all. In this case, they are compiled into the automaton itself, at the expense of more states than what would be required without an assertion.

Variants

Start = { _ }: Match the beginning of text. Specifically, this matches at the starting position of the input.
End = { _ }: Match the end of text. Specifically, this matches at the ending position of the input.
StartLF = { _ }: Match the beginning of a line or the beginning of text. Specifically, this matches at the starting position of the input, or at the position immediately following a \n character.
EndLF = { _ }: Match the end of a line or the end of text. Specifically, this matches at the end position of the input, or at the position immediately preceding a \n character.
StartCRLF = { _ }: Match the beginning of a line or the beginning of text. Specifically, this matches at the starting position of the input, or at the position immediately following either a \r or \n character, but never after a \r when a \n follows.
EndCRLF = { _ }: Match the end of a line or the end of text. Specifically, this matches at the end position of the input, or at the position immediately preceding a \r or \n character, but never before a \n when a \r precedes it.
WordAscii = { _ }: Match an ASCII-only word boundary. That is, this matches a position where the left adjacent character and right adjacent character correspond to a word and non-word or a non-word and word character.
WordAsciiNegate = { _ }: Match an ASCII-only negation of a word boundary.
WordUnicode = { _ }: Match a Unicode-aware word boundary. That is, this matches a position where the left adjacent character and right adjacent character correspond to a word and non-word or a non-word and word character.
WordUnicodeNegate = { _ }: Match a Unicode-aware negation of a word boundary.
WordStartAscii = { _ }: Match the start of an ASCII-only word boundary. That is, this matches a position at either the beginning of the haystack or where the previous character is not a word character and the following character is a word character.
WordEndAscii = { _ }: Match the end of an ASCII-only word boundary. That is, this matches a position at either the end of the haystack or where the previous character is a word character and the following character is not a word character.
WordStartUnicode = { _ }: Match the start of a Unicode word boundary. That is, this matches a position at either the beginning of the haystack or where the previous character is not a word character and the following character is a word character.
WordEndUnicode = { _ }: Match the end of a Unicode word boundary. That is, this matches a position at either the end of the haystack or where the previous character is a word character and the following character is not a word character.
WordStartHalfAscii = { _ }: Match the start half of an ASCII-only word boundary. That is, this matches a position at either the beginning of the haystack or where the previous character is not a word character.
WordEndHalfAscii = { _ }: Match the end half of an ASCII-only word boundary. That is, this matches a position at either the end of the haystack or where the following character is not a word character.
WordStartHalfUnicode = { _ }: Match the start half of a Unicode word boundary. That is, this matches a position at either the beginning of the haystack or where the previous character is not a word character.
WordEndHalfUnicode = { _ }: Match the end half of a Unicode word boundary. That is, this matches a position at either the end of the haystack or where the following character is not a word character.

Implementations

`impl Look`

const fn reversed(self) -> Look

Flip the look-around assertion to its equivalent for reverse searches. For example, StartLF gets translated to EndLF.

Some assertions, such as WordUnicode, remain the same since they match the same positions regardless of the direction of the search.

const fn as_repr(self) -> u32

Return the underlying representation of this look-around enumeration as an integer. Giving the return value to the Look::from_repr constructor is guaranteed to return the same look-around variant that one started with within a semver compatible release of this crate.

const fn from_repr(repr: u32) -> Option<Look>

Given the underlying representation of a Look value, return the corresponding Look value if the representation is valid. Otherwise None is returned.

const fn as_char(self) -> char

Returns a convenient single codepoint representation of this look-around assertion. Each assertion is guaranteed to be represented by a distinct character.

This is useful for succinctly representing a look-around assertion in human friendly but succinct output intended for a programmer working on regex internals.

Trait Implementations

`impl Clone for Look`

fn clone(&self) -> Self

`impl Copy for Look`

`impl Debug for Look`

fn fmt(&self, f: &mut Formatter<'_>) -> Result

`impl Eq for Look`

`impl PartialEq for Look`

fn eq(&self, other: &Self) -> bool

`impl StructuralPartialEq for Look`

Auto Trait Implementations

`impl Freeze for Look`

`impl RefUnwindSafe for Look`

`impl Send for Look`

`impl Sync for Look`

`impl Unpin for Look`

`impl UnsafeUnpin for Look`

`impl UnwindSafe for Look`

Blanket Implementations

`impl<T> Any for Look where T: 'static + ?Sized,`

fn type_id(&self) -> TypeId

`impl<T> Borrow<T> for Look where T: ?Sized,`

fn borrow(&self) -> &T

`impl<T> BorrowMut<T> for Look where T: ?Sized,`

fn borrow_mut(&mut self) -> &mut T

`impl<T> CloneToUninit for Look where T: Clone,`

unsafe fn clone_to_uninit(&self, dest: *mut u8)

`impl<T> From<T> for Look`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T> ToOwned for Look where T: Clone,`

type Owned = T;
fn to_owned(&self) -> T
fn clone_into(&self, target: &mut T)

`impl<T, U> Into for Look where U: From<T>,`

fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

`impl<T, U> TryFrom for Look where U: Into<T>,`

type Error = never;
fn try_from(value: U) -> Result<T, never>

`impl<T, U> TryInto for Look where U: TryFrom<T>,`

type Error = >::Error;
fn try_into(self) -> Result<U, >::Error>