Regex

1 min read

Use a regular expression to find text matching a pattern. Create it once, then test text, extract matches, or replace them. Regex.compile returns an error when the pattern is invalid.

Import: import system.text;

Compile a pattern once, then test, replace, or extract matches. Patterns use familiar regex syntax (\d, groups, lookaround). Regex.compile returns Result<Regex, RegexError>.

Match positions use UTF-16 code units. After an empty match, the next search advances past a complete character, including a surrogate pair, so a global search always progresses.

import system;
import system.text;

fun main() {
    let digits = Regex.compile("\\d+", RegexFlags.Global).unwrap();
    System.println(digits.test("abc123"));
    System.println(digits.replace("a1b2c3", "#"));
}

true
a#b#c#

Flags

Combine with |. Default is RegexFlags.None.

Flag	Meaning
Global	replace / match every occurrence
IgnoreCase	case-insensitive
Multiline	^ / $ at line edges
DotAll	. matches newlines

let re = Regex.compile("hello", RegexFlags.IgnoreCase | RegexFlags.Global).unwrap();

Methods

Call	Meaning
Regex.compile(pattern).unwrap() / Regex.compile(pattern, flags).unwrap()	compile
test(input)	any match?
replace(input, replacement)	substitute ($&, $1, ${name}, $$)
match(input)	string[] of matches
match_info(input)	Option<RegexMatchInfo> with groups and names

Global changes match: without it, the array is the whole match plus each capturing group; with it, every full match is one element.

let digits = Regex.compile("\\d+", RegexFlags.Global).unwrap();
let parts = digits.match("a1b2c3"); // ["1", "2", "3"]

Groups

match_info returns the first match, or None. full is the whole match. groups is each capturing group in order (groups[0] is group 1). Named groups (?<name>...) are looked up with named:

let re = Regex.compile("(?<area>\\d{3})-(?<num>\\d{4})").unwrap();
switch (re.match_info("555-1234")) {
    Some(info) => {
        System.println(info.full); // 555-1234
        System.println(info.named("area").unwrap()); // 555
    }
    None => System.println("no match"),
}

Save the first example as regex.dream and run dream run regex.dream. It prints whether a match exists, then the text with every numeric group replaced by #.

Invalid patterns

Handle patterns supplied by users without unwrapping the result:

import system;
import system.text;

fun main(): void {
    switch (Regex.compile("[unfinished")) {
        Ok(regex) => System.println(regex.test("example")),
        Err(error) => System.println(error.message()),
    }
}

Compilation failures have code EREGEX. Successfully compiled patterns that find no match return false from test or None from match_info.