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String operations.
A string is an immutable data structure that contains a fixed-length sequence of (single-byte) characters. Each character can be accessed in constant time through its index.
Given a string s of length l, we can access each of the l characters of s via its index in the sequence. Indexes start at 0, and we will call an index valid in s if it falls within the range [0...l-1] (inclusive). A position is the point between two characters or at the beginning or end of the string. We call a position valid in s if it falls within the range [0...l] (inclusive). Note that the character at index n is between positions n and n+1.
Two parameters start and len are said to designate a valid substring of s if len >= 0 and start and start+len are valid positions in s.
OCaml strings used to be modifiable in place, for instance via the String.set and String.blit functions described below. This usage is deprecated and only possible when the compiler is put in "unsafe-string" mode by giving the -unsafe-string command-line option (which is currently the default for reasons of backward compatibility). This is done by making the types string and bytes (see module Bytes) interchangeable so that functions expecting byte sequences can also accept strings as arguments and modify them.
All new code should avoid this feature and be compiled with the -safe-string command-line option to enforce the separation between the types string and bytes.
val length : string -> int
Return the length (number of characters) of the given string.
val get : string ->int -> char
String.get s n returns the character at index n in string s. You can also write s.[n] instead of String.get s n.
Raise Invalid_argument if n not a valid index in s.
val make : int ->char -> string
String.make n c returns a fresh string of length n, filled with the character c.
String.iter f s applies function f in turn to all the characters of s. It is equivalent to f s.[0]; f s.[1]; ...; f s.[String.length s - 1]; ().
val iteri : (int ->char -> unit)->string -> unit
Same as String.iter, but the function is applied to the index of the element as first argument (counting from 0), and the character itself as second argument.
since 4.00.0
val map : (char -> char)->string -> string
String.map f s applies function f in turn to all the characters of s (in increasing index order) and stores the results in a new string that is returned.
since 4.00.0
val mapi : (int ->char -> char)->string -> string
String.mapi f s calls f with each character of s and its index (in increasing index order) and stores the results in a new string that is returned.
since 4.02.0
val trim : string -> string
Return a copy of the argument, without leading and trailing whitespace. The characters regarded as whitespace are: ' ', '\012', '\n', '\r', and '\t'. If there is neither leading nor trailing whitespace character in the argument, return the original string itself, not a copy.
since 4.00.0
val escaped : string -> string
Return a copy of the argument, with special characters represented by escape sequences, following the lexical conventions of OCaml. All characters outside the ASCII printable range (32..126) are escaped, as well as backslash and double-quote.
If there is no special character in the argument that needs escaping, return the original string itself, not a copy.
The function Scanf.unescaped is a left inverse of escaped, i.e. Scanf.unescaped (escaped s) = s for any string s (unless escape s fails).
val index_opt : string ->char ->int option
String.index_opt s c returns the index of the first occurrence of character c in string s, or None if c does not occur in s.
since 4.05
val rindex_opt : string ->char ->int option
String.rindex_opt s c returns the index of the last occurrence of character c in string s, or None if c does not occur in s.
since 4.05
val index_from_opt : string ->int ->char ->int option
String.index_from_opt s i c returns the index of the first occurrence of character c in string s after position i or None if c does not occur in s after position i.
String.index_opt s c is equivalent to String.index_from_opt s 0 c. Raise Invalid_argument if i is not a valid position in s.
since 4.05
val rindex_from_opt : string ->int ->char ->int option
String.rindex_from_opt s i c returns the index of the last occurrence of character c in string s before position i+1 or None if c does not occur in s before position i+1.
String.rindex_opt s c is equivalent to String.rindex_from_opt s (String.length s - 1) c.
Raise Invalid_argument if i+1 is not a valid position in s.
since 4.05
val contains : string ->char -> bool
String.contains s c tests if character c appears in the string s.
val contains_from : string ->int ->char -> bool
String.contains_from s start c tests if character c appears in s after position start. String.contains s c is equivalent to String.contains_from s 0 c.
Raise Invalid_argument if start is not a valid position in s.
val rcontains_from : string ->int ->char -> bool
String.rcontains_from s stop c tests if character c appears in s before position stop+1.
Raise Invalid_argument if stop < 0 or stop+1 is not a valid position in s.
val uppercase_ascii : string -> string
Return a copy of the argument, with all lowercase letters translated to uppercase, using the US-ASCII character set.
since 4.03.0
val lowercase_ascii : string -> string
Return a copy of the argument, with all uppercase letters translated to lowercase, using the US-ASCII character set.
since 4.03.0
val capitalize_ascii : string -> string
Return a copy of the argument, with the first character set to uppercase, using the US-ASCII character set.
since 4.03.0
val uncapitalize_ascii : string -> string
Return a copy of the argument, with the first character set to lowercase, using the US-ASCII character set.
The comparison function for strings, with the same specification as Stdlib.compare. Along with the type t, this function compare allows the module String to be passed as argument to the functors Set.Make and Map.Make.
String.split_on_char sep s returns the list of all (possibly empty) substrings of s that are delimited by the sep character.
The function's output is specified by the following invariants:
The list is not empty.
Concatenating its elements using sep as a separator returns a string equal to the input (String.concat (String.make 1 sep)
(String.split_on_char sep s) = s).
No string in the result contains the sep character.