Struct regex::Regex

pub struct Regex(_);

A compiled regular expression for matching Unicode strings.

It is represented as either a sequence of bytecode instructions (dynamic) or as a specialized Rust function (native). It can be used to search, split or replace text. All searching is done with an implicit .*? at the beginning and end of an expression. To force an expression to match the whole string (or a prefix or a suffix), you must use an anchor like ^ or $ (or \A and \z).

While this crate will handle Unicode strings (whether in the regular expression or in the search text), all positions returned are byte indices. Every byte index is guaranteed to be at a Unicode code point boundary.

The lifetimes 'r and 't in this crate correspond to the lifetime of a compiled regular expression and text to search, respectively.

The only methods that allocate new strings are the string replacement methods. All other methods (searching and splitting) return borrowed pointers into the string given.

Examples

Find the location of a US phone number:

let re = Regex::new("[0-9]{3}-[0-9]{3}-[0-9]{4}").unwrap();
let mat = re.find("phone: 111-222-3333").unwrap();
assert_eq!((mat.start(), mat.end()), (7, 19));

Using the std::str::pattern methods with Regex

Note: This section requires that this crate is compiled with the pattern Cargo feature enabled, which requires nightly Rust.

Since Regex implements Pattern, you can use regexes with methods defined on &str. For example, is_match, find, find_iter and split can be replaced with str::contains, str::find, str::match_indices and str::split.

Here are some examples:

let re = Regex::new(r"\d+").unwrap();
let haystack = "a111b222c";

assert!(haystack.contains(&re));
assert_eq!(haystack.find(&re), Some(1));
assert_eq!(haystack.match_indices(&re).collect::<Vec<_>>(),
           vec![(1, "111"), (5, "222")]);
assert_eq!(haystack.split(&re).collect::<Vec<_>>(), vec!["a", "b", "c"]);

Implementations

impl Regex

Core regular expression methods.

pub fn new(re: &str) -> Result<Regex, Error>

Compiles a regular expression. Once compiled, it can be used repeatedly to search, split or replace text in a string.

If an invalid expression is given, then an error is returned.

pub fn is_match(&self, text: &str) -> bool

Returns true if and only if there is a match for the regex in the string given.

It is recommended to use this method if all you need to do is test a match, since the underlying matching engine may be able to do less work.

Example

Test if some text contains at least one word with exactly 13 Unicode word characters:

let text = "I categorically deny having triskaidekaphobia.";
assert!(Regex::new(r"\b\w{13}\b").unwrap().is_match(text));

pub fn find<'t>(&self, text: &'t str) -> Option<Match<'t>>

Returns the start and end byte range of the leftmost-first match in text. If no match exists, then None is returned.

Note that this should only be used if you want to discover the position of the match. Testing the existence of a match is faster if you use is_match.

Example

Find the start and end location of the first word with exactly 13 Unicode word characters:

let text = "I categorically deny having triskaidekaphobia.";
let mat = Regex::new(r"\b\w{13}\b").unwrap().find(text).unwrap();
assert_eq!(mat.start(), 2);
assert_eq!(mat.end(), 15);

pub fn find_iter<'r, 't>(&'r self, text: &'t str) -> Matches<'r, 't>

Returns an iterator for each successive non-overlapping match in text, returning the start and end byte indices with respect to text.

Example

Find the start and end location of every word with exactly 13 Unicode word characters:

let text = "Retroactively relinquishing remunerations is reprehensible.";
for mat in Regex::new(r"\b\w{13}\b").unwrap().find_iter(text) {
    println!("{:?}", mat);
}

pub fn captures<'t>(&self, text: &'t str) -> Option<Captures<'t>>

Returns the capture groups corresponding to the leftmost-first match in text. Capture group 0 always corresponds to the entire match. If no match is found, then None is returned.

You should only use captures if you need access to the location of capturing group matches. Otherwise, find is faster for discovering the location of the overall match.

Examples

Say you have some text with movie names and their release years, like “‘Citizen Kane’ (1941)”. It’d be nice if we could search for text looking like that, while also extracting the movie name and its release year separately.

let re = Regex::new(r"'([^']+)'\s+\((\d{4})\)").unwrap();
let text = "Not my favorite movie: 'Citizen Kane' (1941).";
let caps = re.captures(text).unwrap();
assert_eq!(caps.get(1).unwrap().as_str(), "Citizen Kane");
assert_eq!(caps.get(2).unwrap().as_str(), "1941");
assert_eq!(caps.get(0).unwrap().as_str(), "'Citizen Kane' (1941)");
// You can also access the groups by index using the Index notation.
// Note that this will panic on an invalid index.
assert_eq!(&caps[1], "Citizen Kane");
assert_eq!(&caps[2], "1941");
assert_eq!(&caps[0], "'Citizen Kane' (1941)");

Note that the full match is at capture group 0. Each subsequent capture group is indexed by the order of its opening (.

We can make this example a bit clearer by using named capture groups:

let re = Regex::new(r"'(?P<title>[^']+)'\s+\((?P<year>\d{4})\)")
               .unwrap();
let text = "Not my favorite movie: 'Citizen Kane' (1941).";
let caps = re.captures(text).unwrap();
assert_eq!(caps.name("title").unwrap().as_str(), "Citizen Kane");
assert_eq!(caps.name("year").unwrap().as_str(), "1941");
assert_eq!(caps.get(0).unwrap().as_str(), "'Citizen Kane' (1941)");
// You can also access the groups by name using the Index notation.
// Note that this will panic on an invalid group name.
assert_eq!(&caps["title"], "Citizen Kane");
assert_eq!(&caps["year"], "1941");
assert_eq!(&caps[0], "'Citizen Kane' (1941)");

Here we name the capture groups, which we can access with the name method or the Index notation with a &str. Note that the named capture groups are still accessible with get or the Index notation with a usize.

The 0th capture group is always unnamed, so it must always be accessed with get(0) or [0].

pub fn captures_iter<'r, 't>(&'r self, text: &'t str) -> CaptureMatches<'r, 't>

Returns an iterator over all the non-overlapping capture groups matched in text. This is operationally the same as find_iter, except it yields information about capturing group matches.

Example

We can use this to find all movie titles and their release years in some text, where the movie is formatted like “‘Title’ (xxxx)”:

let re = Regex::new(r"'(?P<title>[^']+)'\s+\((?P<year>\d{4})\)")
               .unwrap();
let text = "'Citizen Kane' (1941), 'The Wizard of Oz' (1939), 'M' (1931).";
for caps in re.captures_iter(text) {
    println!("Movie: {:?}, Released: {:?}",
             &caps["title"], &caps["year"]);
}
// Output:
// Movie: Citizen Kane, Released: 1941
// Movie: The Wizard of Oz, Released: 1939
// Movie: M, Released: 1931

pub fn split<'r, 't>(&'r self, text: &'t str) -> Split<'r, 't>

Returns an iterator of substrings of text delimited by a match of the regular expression. Namely, each element of the iterator corresponds to text that isn’t matched by the regular expression.

This method will not copy the text given.

Example

To split a string delimited by arbitrary amounts of spaces or tabs:

let re = Regex::new(r"[ \t]+").unwrap();
let fields: Vec<&str> = re.split("a b \t  c\td    e").collect();
assert_eq!(fields, vec!["a", "b", "c", "d", "e"]);

pub fn splitn<'r, 't>(&'r self, text: &'t str, limit: usize) -> SplitN<'r, 't>

Returns an iterator of at most limit substrings of text delimited by a match of the regular expression. (A limit of 0 will return no substrings.) Namely, each element of the iterator corresponds to text that isn’t matched by the regular expression. The remainder of the string that is not split will be the last element in the iterator.

This method will not copy the text given.

Example

Get the first two words in some text:

let re = Regex::new(r"\W+").unwrap();
let fields: Vec<&str> = re.splitn("Hey! How are you?", 3).collect();
assert_eq!(fields, vec!("Hey", "How", "are you?"));

pub fn replace<'t, R: Replacer>(&self, text: &'t str, rep: R) -> Cow<'t, str>

Replaces the leftmost-first match with the replacement provided. The replacement can be a regular string (where $N and $name are expanded to match capture groups) or a function that takes the matches’ Captures and returns the replaced string.

If no match is found, then a copy of the string is returned unchanged.

Replacement string syntax

All instances of $name in the replacement text is replaced with the corresponding capture group name.

name may be an integer corresponding to the index of the capture group (counted by order of opening parenthesis where 0 is the entire match) or it can be a name (consisting of letters, digits or underscores) corresponding to a named capture group.

If name isn’t a valid capture group (whether the name doesn’t exist or isn’t a valid index), then it is replaced with the empty string.

The longest possible name is used. e.g., $1a looks up the capture group named 1a and not the capture group at index 1. To exert more precise control over the name, use braces, e.g., ${1}a.

To write a literal $ use $$.

Examples

Note that this function is polymorphic with respect to the replacement. In typical usage, this can just be a normal string:

let re = Regex::new("[^01]+").unwrap();
assert_eq!(re.replace("1078910", ""), "1010");

But anything satisfying the Replacer trait will work. For example, a closure of type |&Captures| -> String provides direct access to the captures corresponding to a match. This allows one to access capturing group matches easily:

let re = Regex::new(r"([^,\s]+),\s+(\S+)").unwrap();
let result = re.replace("Springsteen, Bruce", |caps: &Captures| {
    format!("{} {}", &caps[2], &caps[1])
});
assert_eq!(result, "Bruce Springsteen");

But this is a bit cumbersome to use all the time. Instead, a simple syntax is supported that expands $name into the corresponding capture group. Here’s the last example, but using this expansion technique with named capture groups:

let re = Regex::new(r"(?P<last>[^,\s]+),\s+(?P<first>\S+)").unwrap();
let result = re.replace("Springsteen, Bruce", "$first $last");
assert_eq!(result, "Bruce Springsteen");

Note that using $2 instead of $first or $1 instead of $last would produce the same result. To write a literal $ use $$.

Sometimes the replacement string requires use of curly braces to delineate a capture group replacement and surrounding literal text. For example, if we wanted to join two words together with an underscore:

let re = Regex::new(r"(?P<first>\w+)\s+(?P<second>\w+)").unwrap();
let result = re.replace("deep fried", "${first}_$second");
assert_eq!(result, "deep_fried");

Without the curly braces, the capture group name first_ would be used, and since it doesn’t exist, it would be replaced with the empty string.

Finally, sometimes you just want to replace a literal string with no regard for capturing group expansion. This can be done by wrapping a byte string with NoExpand:

use regex::NoExpand;

let re = Regex::new(r"(?P<last>[^,\s]+),\s+(\S+)").unwrap();
let result = re.replace("Springsteen, Bruce", NoExpand("$2 $last"));
assert_eq!(result, "$2 $last");

pub fn replace_all<'t, R: Replacer>(&self, text: &'t str, rep: R) -> Cow<'t, str>

Replaces all non-overlapping matches in text with the replacement provided. This is the same as calling replacen with limit set to 0.

See the documentation for replace for details on how to access capturing group matches in the replacement string.

pub fn replacen<'t, R: Replacer>(
    &self,
    text: &'t str,
    limit: usize,
    rep: R
) -> Cow<'t, str>

Replaces at most limit non-overlapping matches in text with the replacement provided. If limit is 0, then all non-overlapping matches are replaced.

See the documentation for replace for details on how to access capturing group matches in the replacement string.

impl Regex

Advanced or “lower level” search methods.

pub fn shortest_match(&self, text: &str) -> Option<usize>

Returns the end location of a match in the text given.

This method may have the same performance characteristics as is_match, except it provides an end location for a match. In particular, the location returned may be shorter than the proper end of the leftmost-first match that you would find via Regex::find.

Note that it is not guaranteed that this routine finds the shortest or “earliest” possible match. Instead, the main idea of this API is that it returns the offset at the point at which the internal regex engine has determined that a match has occurred. This may vary depending on which internal regex engine is used, and thus, the offset itself may change.

Example

Typically, a+ would match the entire first sequence of a in some text, but shortest_match can give up as soon as it sees the first a.

let text = "aaaaa";
let pos = Regex::new(r"a+").unwrap().shortest_match(text);
assert_eq!(pos, Some(1));

pub fn shortest_match_at(&self, text: &str, start: usize) -> Option<usize>

Returns the same as shortest_match, but starts the search at the given offset.

The significance of the starting point is that it takes the surrounding context into consideration. For example, the \A anchor can only match when start == 0.

pub fn is_match_at(&self, text: &str, start: usize) -> bool

Returns the same as is_match, but starts the search at the given offset.

The significance of the starting point is that it takes the surrounding context into consideration. For example, the \A anchor can only match when start == 0.

pub fn find_at<'t>(&self, text: &'t str, start: usize) -> Option<Match<'t>>

Returns the same as find, but starts the search at the given offset.

The significance of the starting point is that it takes the surrounding context into consideration. For example, the \A anchor can only match when start == 0.

pub fn captures_at<'t>(&self, text: &'t str, start: usize) -> Option<Captures<'t>>

Returns the same as Regex::captures, but starts the search at the given offset.

The significance of the starting point is that it takes the surrounding context into consideration. For example, the \A anchor can only match when start == 0.

pub fn captures_read<'t>(
    &self,
    locs: &mut CaptureLocations,
    text: &'t str
) -> Option<Match<'t>>

This is like captures, but uses CaptureLocations instead of Captures in order to amortize allocations.

To create a CaptureLocations value, use the Regex::capture_locations method.

This returns the overall match if this was successful, which is always equivalence to the 0th capture group.

pub fn captures_read_at<'t>(
    &self,
    locs: &mut CaptureLocations,
    text: &'t str,
    start: usize
) -> Option<Match<'t>>

Returns the same as captures, but starts the search at the given offset and populates the capture locations given.

The significance of the starting point is that it takes the surrounding context into consideration. For example, the \A anchor can only match when start == 0.

impl Regex

Auxiliary methods.

pub fn as_str(&self) -> &str

Returns the original string of this regex.

pub fn capture_names(&self) -> CaptureNames<'_>

Returns an iterator over the capture names.

pub fn captures_len(&self) -> usize

Returns the number of captures.

pub fn static_captures_len(&self) -> Option<usize>

Returns the total number of capturing groups that appear in every possible match.

If the number of capture groups can vary depending on the match, then this returns None. That is, a value is only returned when the number of matching groups is invariant or “static.”

Note that like Regex::captures_len, this does include the implicit capturing group corresponding to the entire match. Therefore, when a non-None value is returned, it is guaranteed to be at least 1. Stated differently, a return value of Some(0) is impossible.

Example

This shows a few cases where a static number of capture groups is available and a few cases where it is not.

use regex::Regex;

let len = |pattern| {
    Regex::new(pattern).map(|re| re.static_captures_len())
};

assert_eq!(Some(1), len("a")?);
assert_eq!(Some(2), len("(a)")?);
assert_eq!(Some(2), len("(a)|(b)")?);
assert_eq!(Some(3), len("(a)(b)|(c)(d)")?);
assert_eq!(None, len("(a)|b")?);
assert_eq!(None, len("a|(b)")?);
assert_eq!(None, len("(b)*")?);
assert_eq!(Some(2), len("(b)+")?);

pub fn capture_locations(&self) -> CaptureLocations

Returns an empty set of capture locations that can be reused in multiple calls to captures_read or captures_read_at.

Trait Implementations

impl Clone for Regex

fn clone(&self) -> Regex

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Regex

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

Shows the original regular expression.

impl Display for Regex

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

Shows the original regular expression.

impl FromStr for Regex

fn from_str(s: &str) -> Result<Regex, Error>

Attempts to parse a string into a regular expression

type Err = Error

The associated error which can be returned from parsing.