std:regex

The Regex module provides regular expression matching and manipulation.

Import

import { Regex } from "std:regex";

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Creating a Regex

Regex(pattern) creates a regex object. Write patterns as raw strings, r"...", so backslashes reach the regex engine unchanged.

import { Regex } from "std:regex";

let digits = Regex(r"\d+");                     // One or more digits
let email = Regex(r"(?i)[a-z]+@[a-z]+\.com");   // Case-insensitive
print(digits.pattern);  // \d+

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Flags

Set flags inside the pattern:

• (?i) - Case-insensitive matching
• (?m) - Multiline mode: ^ and $ match at line breaks
• (?s) - . also matches newlines

match returns the first match. findAll and replace work on all matches, so no global flag is needed.

Methods

test(input: String): Bool

Returns true if the pattern matches anywhere in the input string.

import { Regex } from "std:regex";

let pattern = Regex(r"\d+");
print(pattern.test("abc123"));  // true
print(pattern.test("abc"));     // false

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match(input: String): Match?

Returns the first match or null if no match found.

import { Regex } from "std:regex";

let pattern = Regex(r"(\w+)@(\w+)");
let m = pattern.match("contact@pleno.dev");
if (m != null) {
    print(m.value);     // contact@pleno
    print(m.groups[0]); // contact
    print(m.groups[1]); // pleno
}

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findAll(input: String): Match[]

Returns an array of all matches.

import { Regex } from "std:regex";

let pattern = Regex(r"\d+");
let matches = pattern.findAll("a1b2c3");
print(matches.length);    // 3
print(matches[0].value);  // 1
print(matches[2].value);  // 3

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replace(input: String, replacement: String): String

Replaces all matches with the replacement string. $1, $2, ... in the replacement insert capture groups.

import { Regex } from "std:regex";

print(Regex(r"\s+").replace("hello   world", " "));         // hello world
print(Regex(r"(\w+)@(\w+)").replace("ann@site", "$2 at $1"));  // site at ann

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split(input: String): String[]

Splits the input string by the pattern.

import { Regex } from "std:regex";

let pattern = Regex(r",\s*");
print(pattern.split("a, b,  c"));  // [a, b, c]

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Static Functions

Regex.isMatch(pattern: String, input: String): Bool

Checks a pattern without creating a regex object.

import { Regex } from "std:regex";

print(Regex.isMatch(r"^\d+$", "123"));  // true

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Regex.replace(pattern: String, input: String, replacement: String): String

Replaces all matches without creating a regex object.

import { Regex } from "std:regex";

print(Regex.replace("o", "foo", "0"));  // f00

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Match Object

The Match object returned by match() and findAll() contains:

Property	Type	Description
value	String	The matched text
index	Int	Start position in input, in bytes
groups	String[]	Captured groups
group(i)	String?	Group i; group(0) is the whole match

A match can also be read like an array: m[0] is the whole match, m[1] the first group, and m.length the number of entries.

import { Regex } from "std:regex";

let m = Regex(r"(\d{4})-(\d{2})-(\d{2})").match("2026-02-03");
print(m.value);     // 2026-02-03
print(m.groups[0]); // 2026
print(m.group(2));  // 02
print(m[3]);        // 03

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