Fails if the value does not equal the expected value.

Fails if the string length is outside [min, max].

Edge case — min > max: the resulting validator is vacuously unsatisfiable (no string length can be both >= min and <= max), so every input fails with error. This is a programmer error rather than a runtime condition; the library does not raise it because rule constructors are pure and never panic. Construct the rule yourself with a sane range, or guard the bounds at the call site (e.g., case min <= max { True -> ...; False -> ... }) when min/max come from configuration or other dynamic input.

Fails if the regex does not find a match anywhere in the string. This is regexp.check semantics — a partial / substring match is enough to pass.

Anchoring is the caller’s responsibility. A pattern like [0-9]+ accepts "abc123def" because the digit run matches somewhere in the input. To require the entire string to match, either anchor the pattern explicitly with ^...$, or reach for matches_fully which enforces full-string semantics regardless of whether the pattern is anchored. The validation use case almost always wants matches_fully; matches is exposed for the cases that genuinely need substring search.

Takes a pre-compiled Regexp so a malformed pattern surfaces as a regexp.from_string error at the call site instead of crashing inside the validator.

Example: import gleam/regexp import dataprep/rules

let assert Ok(re) = regexp.from_string(“^[a-z]+$”) let check = rules.matches(pattern: re, error: InvalidFormat)

For literal patterns where propagating a compile error to the caller is not useful, see matches_string.

Fails if the regex does not match the entire input string. Unlike matches, a partial / substring hit is not enough — the matched substring must equal the whole input. Equivalent to Python’s re.fullmatch semantics.

Anchoring the pattern (^...$) is therefore not required: the validator does the equivalent check itself by comparing the first match’s content against the input. Patterns that already include ^ / $ continue to work; the anchors just become redundant.

Example: import gleam/regexp import dataprep/rules

let assert Ok(re) = regexp.from_string(“[0-9]+”) let check = rules.matches_fully(pattern: re, error: NotANumber)

check(“123”) // Valid(“123”) check(“abc123def”) // Invalid([NotANumber]) – substring match rejected

Fails if the regex does not match the entire input string. Compiles the pattern internally; an invalid pattern panics at construction time with the underlying compile error.

Equivalent to matches_fully but with a literal pattern. Use this for the validation use case — "[0-9]+" will reject "abc123def" rather than accepting it on a substring hit, so the API behaves the way readers usually assume.

Example: import dataprep/rules

let check = rules.matches_fully_string( pattern: “[a-z0-9-]+”, error: InvalidFormat, )

Fails if the string does not match the given regular expression pattern. Compiles the pattern internally; an invalid pattern panics at construction time with the underlying compile error.

Same anchoring footgun as matches: a pattern like "[0-9]+" accepts "abc123def" because the digit run matches somewhere. Anchor explicitly with ^...$ or use matches_fully_string for the validation case.

Use this when the pattern is a literal known at the call site and a compile failure would be a programmer error there is no meaningful recovery from. For dynamically-supplied patterns, use matches together with regexp.from_string so the Result is visible.

Example: import dataprep/rules

let check = rules.matches_string( pattern: “^[a-z0-9-]+$”, error: InvalidFormat, )

Fails if the float exceeds max.

Fails if the int exceeds max.

Fails if the string length exceeds max.

Fails if the float is less than min.

Fails if the int is less than min.

Fails if the string length is less than min.

Fails if the float is negative (less than 0.0).

Fails if the int is negative (less than 0).

Fails if the string is empty or contains only whitespace. Unlike not_empty, this rejects " " and "\t\n". The value is NOT trimmed – it is returned unchanged on success.

Fails if the string is exactly “”. Whitespace-only strings like “ “ pass this check. To reject whitespace-only input, compose with prep.trim() first:

raw |> prep.trim() |> rules.not_empty(MyError)

Fails if the value is not in the allowed list.

Edge case — empty allowed list: a set-membership check against the empty set has no inhabitants, so every input fails with error. This is a programmer error rather than a runtime condition; the library does not raise it because rule constructors are pure and never panic. Either construct the rule with a non-empty allowlist, or guard at the call site (e.g., case allowed { [] -> ...; [_, ..] -> rules.one_of(allowed, e) }) when the allowlist comes from configuration or other dynamic input.