Regex

Regular expressions are available as string methods. When built with RE2 (the default), regex operations are guaranteed O(n) with no risk of catastrophic backtracking. Without RE2, Praia falls back to std::regex.

String regex methods

Method	Description
.test(pattern)	Returns true if the pattern matches anywhere
.match(pattern)	First match map ({match, groups, index}) or nil
.matchAll(pattern)	Array of all match maps
.replacePattern(pattern, replacement)	Replace all matches (supports $1, $2 back-references)

test

"hello123".test("[0-9]+")       // true
"hello".test("[0-9]+")          // false

match

let m = "age: 25".match("(\\w+): (\\d+)")
print(m.match)      // age: 25
print(m.groups)     // ["age", "25"]
print(m.index)      // 0

"hello".match("\\d+")   // nil

matchAll

let nums = "abc123def456".matchAll("\\d+")
for (m in nums) {
    print(m.match, "at", m.index)
}
// 123 at 3
// 456 at 9

replacePattern

"hello   world".replacePattern("\\s+", " ")
// "hello world"

"John Smith".replacePattern("(\\w+) (\\w+)", "$2, $1")
// "Smith, John"

Use .replace() for literal string replacement, .replacePattern() for regex.

Error handling

Invalid regex patterns throw a catchable error:

try {
    "test".test("[invalid")
} catch (err) {
    print(err)      // Invalid regex: ...
}

re grain (advanced regex)

The re grain provides named capture groups, regex split, and escape.

use "re"

Function	Description
re.test(str, pattern)	Returns true if pattern matches
re.find(str, pattern)	First match with groups, named, index (or nil)
re.findAll(str, pattern)	Array of all matches with named maps
re.replace(str, pattern, repl)	Replace all matches
re.split(str, pattern)	Split string by regex pattern
re.escape(str)	Escape special regex characters

Named groups

Use (?<name>...) syntax:

let m = re.find("2026-04-22", "(?<year>\\d{4})-(?<month>\\d{2})-(?<day>\\d{2})")
print(m.named.year)    // "2026"
print(m.named.month)   // "04"
print(m.named.day)     // "22"

split

re.split("one,,two,,,three", ",+")       // ["one", "two", "three"]
re.split("hello world  foo", "\\s+")     // ["hello", "world", "foo"]

escape

let literal = re.escape("file (1).txt")
re.test("file (1).txt", literal)          // true

Practical examples

// Email validation
let email = "ada@example.com"
if (email.test("^[\\w.+-]+@[\\w-]+\\.[\\w.]+$")) {
    print("valid email")
}

// Extract all words
let words = "Hello, World! 123".matchAll("[a-zA-Z]+")
for (w in words) { print(w.match) }

// Clean up whitespace
let clean = "  too   many   spaces  ".strip().replacePattern("\\s+", " ")
print(clean)    // "too many spaces"