path: root/lib/stdlib/doc/src/string.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>string</title>
    <prepared>Robert Virding</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno>1</docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>1996-09-28</date>
    <rev>A</rev>
    <file>string.xml</file>
  </header>
  <module>string</module>
  <modulesummary>String processing functions.</modulesummary>
  <description>
    <p>This module provides functions for string processing.</p>
    <p>A string in this module is represented by <seealso marker="unicode#type-chardata">
    <c>unicode:chardata()</c></seealso>, that is, a list of codepoints,
    binaries with UTF-8-encoded codepoints
    (<em>UTF-8 binaries</em>), or a mix of the two.</p>
    <code>
"abcd"               is a valid string
&lt;&lt;"abcd">>           is a valid string
["abcd"]             is a valid string
&lt;&lt;"abc..åäö"/utf8>>  is a valid string
&lt;&lt;"abc..åäö">>       is NOT a valid string,
                     but a binary with Latin-1-encoded codepoints
[&lt;&lt;"abc">>, "..åäö"] is a valid string
[atom]               is NOT a valid string</code>
    <p>
      This module operates on grapheme clusters. A <em>grapheme cluster</em>
      is a user-perceived character, which can be represented by several
      codepoints.
    </p>
    <code>
"å"  [229] or [97, 778]
"e̊"  [101, 778]</code>
    <p>
      The string length of "ß↑e̊" is 3, even though it is represented by the
      codepoints <c>[223,8593,101,778]</c> or the UTF-8 binary
      <c>&lt;&lt;195,159,226,134,145,101,204,138>></c>.
    </p>
    <p>
      Grapheme clusters for codepoints of class <c>prepend</c>
      and non-modern (or decomposed) Hangul is not handled for performance
      reasons in
      <seealso marker="#find/3"><c>find/3</c></seealso>,
      <seealso marker="#replace/3"><c>replace/3</c></seealso>,
      <seealso marker="#split/2"><c>split/2</c></seealso>,
      <seealso marker="#lexemes/2"><c>split/2</c></seealso> and
      <seealso marker="#trim/3"><c>trim/3</c></seealso>.
    </p>
    <p>
      Splitting and appending strings is to be done on grapheme clusters
      borders.
      There is no verification that the results of appending strings are
      valid or normalized.
    </p>
    <p>
      Most of the functions expect all input to be normalized to one form,
      see for example <seealso marker="unicode#characters_to_nfc_list/1">
      <c>unicode:characters_to_nfc_list/1</c></seealso>.
    </p>
    <p>
      Language or locale specific handling of input is not considered
      in any function.
    </p>
    <p>
      The functions can crash for non-valid input strings. For example,
      the functions expect UTF-8 binaries but not all functions
      verify that all binaries are encoded correctly.
    </p>
    <p>
      Unless otherwise specified the return value type is the same as
      the input type. That is, binary input returns binary output,
      list input returns a list output, and mixed input can return a
      mixed output.</p>
      <code>
1> string:trim("  sarah  ").
"sarah"
2> string:trim(&lt;&lt;"  sarah  ">>).
&lt;&lt;"sarah">>
3> string:lexemes("foo bar", " ").
["foo","bar"]
4> string:lexemes(&lt;&lt;"foo bar">>, " ").
[&lt;&lt;"foo">>,&lt;&lt;"bar">>]</code>
    <p>This module has been reworked in Erlang/OTP 20 to
    handle <seealso marker="unicode#type-chardata">
    <c>unicode:chardata()</c></seealso> and operate on grapheme
    clusters. The <c>old functions</c> that only work on Latin-1 lists as input
    are kept for backwards compatibility reasons but should not be used.
    </p>
  </description>

  <datatypes>
    <datatype>
      <name name="direction"/>
      <name name="grapheme_cluster"/>
      <desc>
        <p>A user-perceived character, consisting of one or more
        codepoints.</p>
      </desc>
    </datatype>
  </datatypes>

  <funcs>

    <func>
      <name name="casefold" arity="1"/>
      <fsummary>Convert a string to a comparable string.</fsummary>
      <desc>
        <p>
	  Converts <c><anno>String</anno></c> to a case-agnostic
	  comparable string. Function <c>casefold/1</c> is preferred
	  over <c>lowercase/1</c> when two strings are to be compared
	  for equality. See also <seealso marker="#equal/4"><c>equal/4</c></seealso>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:casefold("Ω and ẞ SHARP S").</input>
"ω and ss sharp s"</pre>
      </desc>
    </func>

    <func>
      <name name="chomp" arity="1"/>
      <fsummary>Remove trailing end of line control characters.</fsummary>
      <desc>
        <p>
	  Returns a string where any trailing <c>\n</c> or
	  <c>\r\n</c> have been removed from <c><anno>String</anno></c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
182> <input>string:chomp(&lt;&lt;"\nHello\n\n">>).</input>
&lt;&lt;"\nHello">>
183> <input>string:chomp("\nHello\r\r\n").</input>
"\nHello\r"</pre>
      </desc>
    </func>

    <func>
      <name name="equal" arity="2"/>
      <name name="equal" arity="3"/>
      <name name="equal" arity="4"/>
      <fsummary>Test string equality.</fsummary>
      <desc>
        <p>
	  Returns <c>true</c> if <c><anno>A</anno></c> and
          <c><anno>B</anno></c> are equal, otherwise <c>false</c>.
	</p>
	<p>
	  If <c><anno>IgnoreCase</anno></c> is <c>true</c>
	  the function does <seealso marker="#casefold/1">
	  <c>casefold</c>ing</seealso> on the fly before the equality test.
	</p>
	<p>If <c><anno>Norm</anno></c> is not <c>none</c>
	the function applies normalization on the fly before the equality test.
	There are four available normalization forms:
	<seealso marker="unicode#characters_to_nfc_list/1"> <c>nfc</c></seealso>,
	<seealso marker="unicode#characters_to_nfd_list/1"> <c>nfd</c></seealso>,
	<seealso marker="unicode#characters_to_nfkc_list/1"> <c>nfkc</c></seealso>, and
	<seealso marker="unicode#characters_to_nfkd_list/1"> <c>nfkd</c></seealso>.
	</p>
	<p>By default,
	<c><anno>IgnoreCase</anno></c> is <c>false</c> and
	<c><anno>Norm</anno></c> is <c>none</c>.</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:equal("åäö", &lt;&lt;"åäö"/utf8>>).</input>
true
2> <input>string:equal("åäö", unicode:characters_to_nfd_binary("åäö")).</input>
false
3> <input>string:equal("åäö", unicode:characters_to_nfd_binary("ÅÄÖ"), true, nfc).</input>
true</pre>
      </desc>
    </func>

    <func>
      <name name="find" arity="2"/>
      <name name="find" arity="3"/>
      <fsummary>Find start of substring.</fsummary>
      <desc>
        <p>
	  Removes anything before <c><anno>SearchPattern</anno></c> in <c><anno>String</anno></c>
	  and returns the remainder of the string or <c>nomatch</c> if <c><anno>SearchPattern</anno></c> is not
	  found.
          <c><anno>Dir</anno></c>, which can be <c>leading</c> or
	  <c>trailing</c>, indicates from which direction characters
	  are to be searched.
        </p>
	<p>
          By default, <c><anno>Dir</anno></c> is <c>leading</c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:find("ab..cd..ef", ".").</input>
"..cd..ef"
2> <input>string:find(&lt;&lt;"ab..cd..ef">>, "..", trailing).</input>
&lt;&lt;"..ef">>
3> <input>string:find(&lt;&lt;"ab..cd..ef">>, "x", leading).</input>
nomatch
4> <input>string:find("ab..cd..ef", "x", trailing).</input>
nomatch</pre>
      </desc>
    </func>

    <func>
      <name name="is_empty" arity="1"/>
      <fsummary>Check if the string is empty.</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>String</anno></c> is the
        empty string, otherwise <c>false</c>.</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:is_empty("foo").</input>
false
2> <input>string:is_empty(["",&lt;&lt;>>]).</input>
true</pre>
      </desc>
    </func>

    <func>
      <name name="length" arity="1"/>
      <fsummary>Calculate length of the string.</fsummary>
      <desc>
        <p>
	  Returns the number of grapheme clusters in <c><anno>String</anno></c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:length("ß↑e̊").</input>
3
2> <input>string:length(&lt;&lt;195,159,226,134,145,101,204,138>>).</input>
3</pre>
      </desc>
    </func>

    <func>
      <name name="lexemes" arity="2"/>
      <fsummary>Split string into lexemes.</fsummary>
      <desc>
	<p>
	  Returns a list of lexemes in <c><anno>String</anno></c>, separated
          by the grapheme clusters in <c><anno>SeparatorList</anno></c>.
	</p>
	<p>
	  Notice that, as shown in this example, two or more
          adjacent separator graphemes clusters in <c><anno>String</anno></c>
          are treated as one. That is, there are no empty
          strings in the resulting list of lexemes.
	  See also <seealso marker="#split/3"><c>split/3</c></seealso> which returns
	  empty strings.
	</p>
	<p>Notice that <c>[$\r,$\n]</c> is one grapheme cluster.</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:lexemes("abc de̊fxxghix jkl\r\nfoo", "x e" ++ [[$\r,$\n]]).</input>
["abc","de̊f","ghi","jkl","foo"]
2> <input>string:lexemes(&lt;&lt;"abc de̊fxxghix jkl\r\nfoo"/utf8>>, "x e" ++ [$\r,$\n]).</input>
[&lt;&lt;"abc">>,&lt;&lt;"de̊f"/utf8>>,&lt;&lt;"ghi">>,&lt;&lt;"jkl\r\nfoo">>]</pre>
      </desc>
    </func>

    <func>
      <name name="lowercase" arity="1"/>
      <fsummary>Convert a string to lowercase</fsummary>
      <desc>
        <p>
	  Converts <c><anno>String</anno></c> to lowercase.
	</p>
	<p>
	  Notice that function <seealso marker="#casefold/1"><c>casefold/1</c></seealso>
	  should be used when converting a string to
	  be tested for equality.
	</p>
	<p><em>Example:</em></p>
	<pre>
2> <input>string:lowercase(string:uppercase("Michał")).</input>
"michał"</pre>
      </desc>
    </func>

    <func>
      <name name="next_codepoint" arity="1"/>
      <fsummary>Pick the first codepoint.</fsummary>
      <desc>
        <p>
	  Returns the first codepoint in <c><anno>String</anno></c>
	  and the rest of <c><anno>String</anno></c> in the tail. Returns
	  an empty list if <c><anno>String</anno></c> is empty or an
	  <c>{error, String}</c> tuple if the next byte is invalid.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:next_codepoint(unicode:characters_to_binary("e̊fg")).</input>
[101|&lt;&lt;"̊fg"/utf8>>]</pre>
      </desc>
    </func>

    <func>
      <name name="next_grapheme" arity="1"/>
      <fsummary>Pick the first grapheme cluster.</fsummary>
      <desc>
        <p>
	  Returns the first grapheme cluster in <c><anno>String</anno></c>
	  and the rest of <c><anno>String</anno></c> in the tail. Returns
	  an empty list if <c><anno>String</anno></c> is empty or an
	  <c>{error, String}</c> tuple if the next byte is invalid.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:next_grapheme(unicode:characters_to_binary("e̊fg")).</input>
["e̊"|&lt;&lt;"fg">>]</pre>
      </desc>
    </func>

    <func>
      <name name="nth_lexeme" arity="3"/>
      <fsummary>Pick the nth lexeme.</fsummary>
      <desc>
	<p>Returns lexeme number <c><anno>N</anno></c> in
	<c><anno>String</anno></c>, where lexemes are separated by
	the grapheme clusters in <c><anno>SeparatorList</anno></c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:nth_lexeme("abc.de̊f.ghiejkl", 3, ".e").</input>
"ghi"</pre>
      </desc>
    </func>

    <func>
      <name name="pad" arity="2"/>
      <name name="pad" arity="3"/>
      <name name="pad" arity="4"/>
      <fsummary>Pad a string to given length.</fsummary>
      <desc>
        <p>
	  Pads <c><anno>String</anno></c> to <c><anno>Length</anno></c> with
	  grapheme cluster <c><anno>Char</anno></c>.
	  <c><anno>Dir</anno></c>, which can be <c>leading</c>, <c>trailing</c>,
	  or <c>both</c>, indicates where the padding should be added.
	</p>
	<p>By default, <c><anno>Char</anno></c> is <c>$\s</c> and
	<c><anno>Dir</anno></c> is <c>trailing</c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:pad(&lt;&lt;"He̊llö"/utf8>>, 8).</input>
[&lt;&lt;72,101,204,138,108,108,195,182>>,32,32,32]
2> <input>io:format("'~ts'~n",[string:pad("He̊llö", 8, leading)]).</input>
'   He̊llö'
3> <input>io:format("'~ts'~n",[string:pad("He̊llö", 8, both)]).</input>
' He̊llö  '</pre>
      </desc>
    </func>

    <func>
      <name name="prefix" arity="2"/>
      <fsummary>Remove prefix from string.</fsummary>
      <desc>
        <p>
	  If <c><anno>Prefix</anno></c> is the prefix of
	  <c><anno>String</anno></c>, removes it and returns the
	  remainder of <c><anno>String</anno></c>, otherwise returns
	  <c>nomatch</c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:prefix(&lt;&lt;"prefix of string">>, "pre").</input>
&lt;&lt;"fix of string">>
2> <input>string:prefix("pre", "prefix").</input>
nomatch</pre>
      </desc>
    </func>

    <func>
      <name name="replace" arity="3"/>
      <name name="replace" arity="4"/>
      <fsummary>Replace a pattern in string.</fsummary>
      <desc>
        <p>
	  Replaces <c><anno>SearchPattern</anno></c> in <c><anno>String</anno></c>
	  with <c><anno>Replacement</anno></c>.
	  <c><anno>Where</anno></c>, default <c>leading</c>, indicates whether
	  the <c>leading</c>, the <c>trailing</c> or <c>all</c> encounters of
	  <c><anno>SearchPattern</anno></c> are to be replaced.
	</p>
	<p>Can be implemented as:</p>
	<pre>lists:join(Replacement, split(String, SearchPattern, Where)).</pre>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:replace(&lt;&lt;"ab..cd..ef">>, "..", "*").</input>
[&lt;&lt;"ab">>,"*",&lt;&lt;"cd..ef">>]
2> <input>string:replace(&lt;&lt;"ab..cd..ef">>, "..", "*", all).</input>
[&lt;&lt;"ab">>,"*",&lt;&lt;"cd">>,"*",&lt;&lt;"ef">>]</pre>
      </desc>
    </func>

    <func>
      <name name="reverse" arity="1"/>
      <fsummary>Reverses a string</fsummary>
      <desc>
        <p>
	  Returns the reverse list of the grapheme clusters in <c><anno>String</anno></c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> Reverse = <input>string:reverse(unicode:characters_to_nfd_binary("ÅÄÖ")).</input>
[[79,776],[65,776],[65,778]]
2> <input>io:format("~ts~n",[Reverse]).</input>
ÖÄÅ</pre>
      </desc>
    </func>

    <func>
      <name name="slice" arity="2"/>
      <name name="slice" arity="3"/>
      <fsummary>Extract a part of string</fsummary>
      <desc>
	<p>Returns a substring of <c><anno>String</anno></c> of
	at most <c><anno>Length</anno></c> grapheme clusters, starting at position
	<c><anno>Start</anno></c>.</p>
	<p>By default, <c><anno>Length</anno></c> is <c>infinity</c>.</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:slice(&lt;&lt;"He̊llö Wörld"/utf8>>, 4).</input>
&lt;&lt;"ö Wörld"/utf8>>
2> <input>string:slice(["He̊llö ", &lt;&lt;"Wörld"/utf8>>], 4,4).</input>
"ö Wö"
3> <input>string:slice(["He̊llö ", &lt;&lt;"Wörld"/utf8>>], 4,50).</input>
"ö Wörld"</pre>
      </desc>
    </func>

    <func>
      <name name="split" arity="2"/>
      <name name="split" arity="3"/>
      <fsummary>Split a string into substrings.</fsummary>
      <desc>
        <p>
	  Splits <c><anno>String</anno></c> where <c><anno>SearchPattern</anno></c>
	  is encountered and return the remaining parts.
	  <c><anno>Where</anno></c>, default <c>leading</c>, indicates whether
	  the <c>leading</c>, the <c>trailing</c> or <c>all</c> encounters of
	  <c><anno>SearchPattern</anno></c> will split <c><anno>String</anno></c>.
	</p>
	<p><em>Example:</em></p>
	<pre>
0> <input>string:split("ab..bc..cd", "..").</input>
["ab","bc..cd"]
1> <input>string:split(&lt;&lt;"ab..bc..cd">>, "..", trailing).</input>
[&lt;&lt;"ab..bc">>,&lt;&lt;"cd">>]
2> <input>string:split(&lt;&lt;"ab..bc....cd">>, "..", all).</input>
[&lt;&lt;"ab">>,&lt;&lt;"bc">>,&lt;&lt;>>,&lt;&lt;"cd">>]</pre>
      </desc>
    </func>

    <func>
      <name name="take" arity="2"/>
      <name name="take" arity="3"/>
      <name name="take" arity="4"/>
      <fsummary>Take leading or trailing parts.</fsummary>
      <desc>
        <p>Takes characters from <c><anno>String</anno></c> as long as
        the characters are members of set <c><anno>Characters</anno></c>
	or the complement of set <c><anno>Characters</anno></c>.
        <c><anno>Dir</anno></c>,
        which can be <c>leading</c> or <c>trailing</c>, indicates from
        which direction characters are to be taken.
	</p>
	<p><em>Example:</em></p>
	<pre>
5> <input>string:take("abc0z123", lists:seq($a,$z)).</input>
{"abc","0z123"}
6> <input>string:take(&lt;&lt;"abc0z123">>, lists:seq($0,$9), true, leading).</input>
{&lt;&lt;"abc">>,&lt;&lt;"0z123">>}
7> <input>string:take("abc0z123", lists:seq($0,$9), false, trailing).</input>
{"abc0z","123"}
8> <input>string:take(&lt;&lt;"abc0z123">>, lists:seq($a,$z), true, trailing).</input>
{&lt;&lt;"abc0z">>,&lt;&lt;"123">>}</pre>
      </desc>
    </func>

    <func>
      <name name="titlecase" arity="1"/>
      <fsummary>Convert a string to titlecase.</fsummary>
      <desc>
        <p>
	  Converts <c><anno>String</anno></c> to titlecase.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:titlecase("ß is a SHARP s").</input>
"Ss is a SHARP s"</pre>
      </desc>
    </func>

    <func>
      <name name="to_float" arity="1"/>
      <fsummary>Return a float whose text representation is the integers
        (ASCII values) of a string.</fsummary>
      <desc>
        <p>Argument <c><anno>String</anno></c> is expected to start with a
          valid text represented float (the digits are ASCII values).
          Remaining characters in the string after the float are returned in
          <c><anno>Rest</anno></c>.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>{F1,Fs} = string:to_float("1.0-1.0e-1"),</input>
> <input>{F2,[]} = string:to_float(Fs),</input>
> <input>F1+F2.</input>
0.9
> <input>string:to_float("3/2=1.5").</input>
{error,no_float}
> <input>string:to_float("-1.5eX").</input>
{-1.5,"eX"}</pre>
      </desc>
    </func>

    <func>
      <name name="to_integer" arity="1"/>
      <fsummary>Return an integer whose text representation is the integers
        (ASCII values) of a string.</fsummary>
      <desc>
        <p>Argument <c><anno>String</anno></c> is expected to start with a
          valid text represented integer (the digits are ASCII values).
          Remaining characters in the string after the integer are returned in
          <c><anno>Rest</anno></c>.</p>
        <p><em>Example:</em></p>
        <pre>
> <input>{I1,Is} = string:to_integer("33+22"),</input>
> <input>{I2,[]} = string:to_integer(Is),</input>
> <input>I1-I2.</input>
11
> <input>string:to_integer("0.5").</input>
{0,".5"}
> <input>string:to_integer("x=2").</input>
{error,no_integer}</pre>
      </desc>
    </func>

    <func>
      <name name="to_graphemes" arity="1"/>
      <fsummary>Convert a string to a list of grapheme clusters.</fsummary>
      <desc>
        <p>
	  Converts <c><anno>String</anno></c> to a list of grapheme clusters.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:to_graphemes("ß↑e̊").</input>
[223,8593,[101,778]]
2> <input>string:to_graphemes(&lt;&lt;"ß↑e̊"/utf8>>).</input>
[223,8593,[101,778]]</pre>
      </desc>
    </func>

    <func>
      <name name="trim" arity="1"/>
      <name name="trim" arity="2"/>
      <name name="trim" arity="3"/>
      <fsummary>Trim leading or trailing, or both, characters.</fsummary>
      <desc>
	<p>
	  Returns a string, where leading or trailing, or both,
	  <c><anno>Characters</anno></c> have been removed.
	  <c><anno>Dir</anno></c> which can be <c>leading</c>, <c>trailing</c>,
	  or <c>both</c>, indicates from which direction characters
	  are to be removed.
	</p>
	<p> Default <c><anno>Characters</anno></c> is the set of
	nonbreakable whitespace codepoints, defined as
	Pattern_White_Space in
	<url href="http://unicode.org/reports/tr31/">Unicode Standard Annex #31</url>.
	<c>By default, <anno>Dir</anno></c> is <c>both</c>.
	</p>
	<p>
	  Notice that <c>[$\r,$\n]</c> is one grapheme cluster according
	  to the Unicode Standard.
	</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:trim("\t  Hello  \n").</input>
"Hello"
2> <input>string:trim(&lt;&lt;"\t  Hello  \n">>, leading).</input>
&lt;&lt;"Hello  \n">>
3> <input>string:trim(&lt;&lt;".Hello.\n">>, trailing, "\n.").</input>
&lt;&lt;".Hello">></pre>
      </desc>
    </func>

    <func>
      <name name="uppercase" arity="1"/>
      <fsummary>Convert a string to uppercase.</fsummary>
      <desc>
        <p>
	  Converts <c><anno>String</anno></c> to uppercase.
	</p>
	<p>See also <seealso marker="#titlecase/1"><c>titlecase/1</c></seealso>.</p>
	<p><em>Example:</em></p>
	<pre>
1> <input>string:uppercase("Michał").</input>
"MICHAŁ"</pre>
      </desc>
    </func>

  </funcs>
</erlref>