Enum Look

pub enum Look {
    Start = 1,
    End = 2,
    StartLF = 4,
    EndLF = 8,
    StartCRLF = 16,
    EndCRLF = 32,
    WordAscii = 64,
    WordAsciiNegate = 128,
    WordUnicode = 256,
    WordUnicodeNegate = 512,
    WordStartAscii = 1_024,
    WordEndAscii = 2_048,
    WordStartUnicode = 4_096,
    WordEndUnicode = 8_192,
    WordStartHalfAscii = 16_384,
    WordEndHalfAscii = 32_768,
    WordStartHalfUnicode = 65_536,
    WordEndHalfUnicode = 131_072,
}

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 = 1

Match the beginning of text. Specifically, this matches at the starting position of the input.

End = 2

Match the end of text. Specifically, this matches at the ending position of the input.

StartLF = 4

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 = 8

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 = 16

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 = 32

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 = 64

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 = 128

Match an ASCII-only negation of a word boundary.

WordUnicode = 256

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 = 512

Match a Unicode-aware negation of a word boundary.

WordStartAscii = 1_024

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 = 2_048

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 = 4_096

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 = 8_192

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 = 16_384

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 = 32_768

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 = 65_536

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 = 131_072

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

pub 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.

pub 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.

pub 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.

pub 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) -> Look

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Look

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

Formats the value using the given formatter.

impl PartialEq for Look

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for Look

impl Eq for Look

impl StructuralPartialEq for Look