Str(3)

okay bestie, welcome to Str(3) from the OCaml library — the regex-and-string-processing module that is about to become your new hyperfixation. 💖

🌸 NAME 🌸

Str — regular expressions and high-level string processing. No cap, that is the whole vibe: regexes plus the high-level string stuff. ✨

🎀 Module 🎀

Module Str

✨ Documentation ✨

Module Str : sig end

Regular expressions and high-level string processing. Yes, the description said it twice; we love consistency. 🫶

💖 Regular expressions 💖

The Str library provides regular expressions on sequences of bytes. It is, in general, unsuitable to match Unicode characters. So like, byte-by-byte energy, not Unicode glamour. 🫶

type regexp — the type of compiled regular expressions. This is the main-character type you will be passing around. 👑

• val regexp : string -> regexp — Compile a regular expression. The following constructs are recognized:
  • . — Matches any character except newline.
  • * (postfix) — Matches the preceding expression zero, one or several times.
  • + (postfix) — Matches the preceding expression one or several times.
  • ? (postfix) — Matches the preceding expression once or not at all.
  • [..] — Character set. Ranges are denoted with -, as in [a-z]. An initial ^, as in [^0-9], complements the set. To include a ] character in a set, make it the first character of the set. To include a - character in a set, make it the first or the last character of the set.
  • ^ — Matches at beginning of line: either at the beginning of the matched string, or just after a '\n' character.
  • $ — Matches at end of line: either at the end of the matched string, or just before a '\n' character.
  • \| (infix) — Alternative between two expressions.
  • \(..\) — Grouping and naming of the enclosed expression.
  • \1 — The text matched by the first \(...\) expression (\2 for the second expression, and so on up to \9).
  • \b — Matches word boundaries.
  • \ — Quotes special characters. The special characters are $^\.*+?[] .

In regular expressions you will often use backslash characters; it's easier to use a quoted string literal {|...|} to avoid having to escape backslashes.

For example, the following expression:

let r = Str.regexp {|hello \([A-Za-z]+\)|} in
      Str.replace_first r {|\1|} "hello world"

returns the string "world" .

If you want a regular expression that matches a literal backslash character, you need to double it: Str.regexp {|\\|} .

You can use regular string literals "..." too, however you will have to escape backslashes. The example above can be rewritten with a regular string literal as:

let r = Str.regexp "hello \\([A-Za-z]+\\)" in
      Str.replace_first r "\\1" "hello world"

And the regular expression for matching a backslash becomes a quadruple backslash: Str.regexp "\\\\" .

• val regexp_case_fold : string -> regexp — Same as regexp, but the compiled expression will match text in a case-insensitive way: uppercase and lowercase letters will be considered equivalent.
• val quote : string -> string — Str.quote s returns a regexp string that matches exactly s and nothing else. periodt.
• val regexp_string : string -> regexp — Str.regexp_string s returns a regular expression that matches exactly s and nothing else.
• val regexp_string_case_fold : string -> regexp — Str.regexp_string_case_fold is similar to Str.regexp_string, but the regexp matches in a case-insensitive way.

🦋 String matching and searching 🦋

• val string_match : regexp -> string -> int -> bool — string_match r s start tests whether a substring of s that starts at position start matches the regular expression r. The first character of a string has position 0, as usual.
• val search_forward : regexp -> string -> int -> int — search_forward r s start searches the string s for a substring matching the regular expression r. The search starts at position start and proceeds towards the end of the string. Return the position of the first character of the matched substring.

Raises Not_found if no substring matches.

• val search_backward : regexp -> string -> int -> int — search_backward r s last searches the string s for a substring matching the regular expression r. The search first considers substrings that start at position last and proceeds towards the beginning of string. Return the position of the first character of the matched substring.

Raises Not_found if no substring matches.

• val string_partial_match : regexp -> string -> int -> bool — Similar to Str.string_match, but also returns true if the argument string is a prefix of a string that matches. This includes the case of a true complete match.
• val matched_string : string -> string — matched_string s returns the substring of s that was matched by the last call to one of the following matching or searching functions:
  • Str.string_match
  • Str.search_forward
  • Str.search_backward
  • Str.string_partial_match
  • Str.global_substitute
  • Str.substitute_first

provided that none of the following functions was called in between:

• Str.global_replace
• Str.replace_first
• Str.split
• Str.bounded_split
• Str.split_delim
• Str.bounded_split_delim
• Str.full_split
• Str.bounded_full_split

Note: in the case of global_substitute and substitute_first, a call to matched_string is only valid within the subst argument, not after global_substitute or substitute_first returns.

⚠️ Vibe check: The user must make sure that the parameter s is the same string that was passed to the matching or searching function.

• val match_beginning : unit -> int — match_beginning() returns the position of the first character of the substring that was matched by the last call to a matching or searching function (see Str.matched_string for details).
• val match_end : unit -> int — match_end() returns the position of the character following the last character of the substring that was matched by the last call to a matching or searching function (see Str.matched_string for details).
• val matched_group : int -> string -> string — matched_group n s returns the substring of s that was matched by the *n*th group \(...\) of the regular expression that was matched by the last call to a matching or searching function (see Str.matched_string for details). When n is 0, it returns the substring matched by the whole regular expression. The user must make sure that the parameter s is the same string that was passed to the matching or searching function.

Raises Not_found if the *n*th group of the regular expression was not matched. This can happen with groups inside alternatives \|, options ? or repetitions *. For instance, the empty string will match \(a\)*, but matched_group 1 "" will raise Not_found because the first group itself was not matched.

• val group_beginning : int -> int — group_beginning n returns the position of the first character of the substring that was matched by the *n*th group of the regular expression that was matched by the last call to a matching or searching function (see Str.matched_string for details).

Raises Not_found if the *n*th group of the regular expression was not matched.

Raises Invalid_argument if there are fewer than n groups in the regular expression.

• val group_end : int -> int — group_end n returns the position of the character following the last character of substring that was matched by the *n*th group of the regular expression that was matched by the last call to a matching or searching function (see Str.matched_string for details).

Raises Not_found if the *n*th group of the regular expression was not matched.

Raises Invalid_argument if there are fewer than n groups in the regular expression.

🌈 Replacement 🌈

• val global_replace : regexp -> string -> string -> string — global_replace regexp templ s returns a string identical to s, except that all substrings of s that match regexp have been replaced by templ. The replacement template templ can contain \1, \2, etc; these sequences will be replaced by the text matched by the corresponding group in the regular expression. \0 stands for the text matched by the whole regular expression.
• val replace_first : regexp -> string -> string -> string — Same as Str.global_replace, except that only the first substring matching the regular expression is replaced.
• val global_substitute : regexp -> (string -> string) -> string -> string — global_substitute regexp subst s returns a string identical to s, except that all substrings of s that match regexp have been replaced by the result of function subst. The function subst is called once for each matching substring, and receives s (the whole text) as argument.
• val substitute_first : regexp -> (string -> string) -> string -> string — Same as Str.global_substitute, except that only the first substring matching the regular expression is replaced.
• val replace_matched : string -> string -> string — replace_matched repl s returns the replacement text repl in which \1, \2, etc. have been replaced by the text matched by the corresponding groups in the regular expression that was matched by the last call to a matching or searching function (see Str.matched_string for details). s must be the same string that was passed to the matching or searching function.

🍓 Splitting 🍓

• val split : regexp -> string -> string list — split r s splits s into substrings, taking as delimiters the substrings that match r, and returns the list of substrings. For instance, split (regexp "[ \t]+") s splits s into blank-separated words. An occurrence of the delimiter at the beginning or at the end of the string is ignored.
• val bounded_split : regexp -> string -> int -> string list — Same as Str.split, but splits into at most n substrings, where n is the extra integer parameter.
• val split_delim : regexp -> string -> string list — Same as Str.split but occurrences of the delimiter at the beginning and at the end of the string are recognized and returned as empty strings in the result. For instance, split_delim (regexp " ") " abc " returns [""; "abc"; ""], while split with the same arguments returns ["abc"].
• val bounded_split_delim : regexp -> string -> int -> string list — Same as Str.bounded_split, but occurrences of the delimiter at the beginning and at the end of the string are recognized and returned as empty strings in the result.

type split_result — the split result type, and it is giving tagged-union realness:

type split_result =
  | Text of string
  | Delim of string

• val full_split : regexp -> string -> split_result list — Same as Str.split_delim, but returns the delimiters as well as the substrings contained between delimiters. The former are tagged Delim in the result list; the latter are tagged Text. For instance, full_split (regexp "[{}]") "{ab}" returns [Delim "{"; Text "ab"; Delim "}"].
• val bounded_full_split : regexp -> string -> int -> split_result list — Same as Str.bounded_split_delim, but returns the delimiters as well as the substrings contained between delimiters. The former are tagged Delim in the result list; the latter are tagged Text.

✨ Extracting substrings ✨

okay bestie, this is the little substring buffet 🍓 — four functions, all returning strings, no cap.

🎀 string_before

val string_before : string -> int -> string

string_before s n gives you the substring made of every character of s that comes before position n — and it excludes the character at position n itself. 💅

🦋 string_after

val string_after : string -> int -> string

string_after s n gives you the substring made of every character of s that follows position n — and it includes the character at position n. iconic.

🌸 first_chars

val first_chars : string -> int -> string

first_chars s n gives you the first n characters of s ✨ — literally the same function as Str.string_before, iykyk.

👑 last_chars

val last_chars : string -> int -> string

last_chars s n gives you the last n characters of s 🫶.

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mwah 💋 — this page is OCamldoc, Str(3), dated 2026-06-22. periodt.