std.strings

import { Aside } from ‘@astrojs/starlight/components’;

std.strings provides byte-oriented operations on string values: searching, trimming, case conversion, replacement, splitting and joining. It is the text-layer counterpart to std.bytes, which provides the same API over mutable []byte.

All operations treat strings as sequences of bytes, not runes. Case conversion (`to_upper`, `to_lower`) and whitespace detection (`trim_space`) handle ASCII only; Unicode-aware variants will live in a future `std.unicode` module.

Every function that produces a string allocates a fresh buffer — Sysl strings are immutable and there is no zero-copy substring view today. Operations that don’t change the string (like contains) don’t allocate.

import std.strings.*

Searching

Function	Signature	Behaviour
has_prefix(s, prefix)	(string, string) -> bool	Empty string is a prefix of every string
has_suffix(s, suffix)	(string, string) -> bool	Empty string is a suffix of every string
index(s, sub)	(string, string) -> int	Byte offset of first occurrence, or -1
last_index(s, sub)	(string, string) -> int	Byte offset of last occurrence, or -1
contains(s, sub)	(string, string) -> bool	Convenience around index >= 0
count(s, sub)	(string, string) -> int	Non-overlapping occurrences; empty sub returns len(s) + 1 (Go parity)
index_byte(s, b)	(string, byte) -> int	First byte offset where s[i] == b
last_index_byte(s, b)	(string, byte) -> int	Last byte offset where s[i] == b

Slicing

Function	Signature	Behaviour
substring(s, start, end)	(string, int, int) -> string	Half-open range [start, end), traps on out-of-bounds

Trimming

Function	Signature	Behaviour
trim_space(s)	(string) -> string	Strip ASCII whitespace from both ends
trim_left(s, cutset)	(string, string) -> string	Strip any leading byte that appears in cutset
trim_right(s, cutset)	(string, string) -> string	Strip any trailing byte that appears in cutset
trim(s, cutset)	(string, string) -> string	trim_left then trim_right

Case conversion (ASCII)

Function	Signature
to_upper(s)	(string) -> string
to_lower(s)	(string) -> string

Replacement and repetition

Function	Signature	Behaviour
replace_all(s, old, new)	(string, string, string) -> string	Every non-overlapping occurrence
repeat(s, count)	(string, int) -> string	Traps on negative count

Splitting and joining

Function	Signature	Behaviour
split(s, sep)	(string, string) -> []string	Splits on every occurrence of sep
fields(s)	(string) -> []string	Splits on runs of ASCII whitespace
join(xs, sep)	([]string, string) -> string	Joins slice with sep between elements

Worked example

import std.strings.*

main() -> int
    val line = "  user=alice   role=admin  "
    val kvs  = fields(trim_space(line))

    for kv in kvs
        val eq = index_byte(kv, '=')
        if eq >= 0
            val k = substring(kv, 0, eq)
            val v = substring(kv, eq + 1, len(kv))
            puts(s"$k -> $v")

    0

Output:

user -> alice
role -> admin

See also

• std.builder — growable string builder used to assemble result strings inside the module.
• std.strconv — parse integers, format numbers, quote strings.
• std.regex — when you need real pattern matching instead of substring search.