path: root/Help/command/string.rst

string
------

String operations.

Synopsis
^^^^^^^^

.. parsed-literal::

  `Search and Replace`_
    string(`FIND`_ <string> <substring> <out-var> [...])
    string(`REPLACE`_ <match-string> <replace-string> <out-var> <input>...)

  `Regular Expressions`_
    string(`REGEX MATCH`_ <match-regex> <out-var> <input>...)
    string(`REGEX MATCHALL`_ <match-regex> <out-var> <input>...)
    string(`REGEX REPLACE`_ <match-regex> <replace-expr> <out-var> <input>...)

  `Manipulation`_
    string(`APPEND`_ <string-var> [<input>...])
    string(`PREPEND`_ <string-var> [<input>...])
    string(`CONCAT`_ <out-var> [<input>...])
    string(`JOIN`_ <glue> <out-var> [<input>...])
    string(`TOLOWER`_ <string> <out-var>)
    string(`TOUPPER`_ <string> <out-var>)
    string(`LENGTH`_ <string> <out-var>)
    string(`SUBSTRING`_ <string> <begin> <length> <out-var>)
    string(`STRIP`_ <string> <out-var>)
    string(`GENEX_STRIP`_ <string> <out-var>)
    string(`REPEAT`_ <string> <count> <out-var>)

  `Comparison`_
    string(`COMPARE`_ <op> <string1> <string2> <out-var>)

  `Hashing`_
    string(`\<HASH\> <HASH_>`_ <out-var> <input>)

  `Generation`_
    string(`ASCII`_ <number>... <out-var>)
    string(`CONFIGURE`_ <string> <out-var> [...])
    string(`MAKE_C_IDENTIFIER`_ <string> <out-var>)
    string(`RANDOM`_ [<option>...] <out-var>)
    string(`TIMESTAMP`_ <out-var> [<format string>] [UTC])
    string(`UUID`_ <out-var> ...)

Search and Replace
^^^^^^^^^^^^^^^^^^

.. _FIND:

.. code-block:: cmake

  string(FIND <string> <substring> <output_variable> [REVERSE])

Return the position where the given ``<substring>`` was found in
the supplied ``<string>``.  If the ``REVERSE`` flag was used, the command will
search for the position of the last occurrence of the specified
``<substring>``.  If the ``<substring>`` is not found, a position of -1 is
returned.

The ``string(FIND)`` subcommand treats all strings as ASCII-only characters.
The index stored in ``<output_variable>`` will also be counted in bytes,
so strings containing multi-byte characters may lead to unexpected results.

.. _REPLACE:

.. code-block:: cmake

  string(REPLACE <match_string>
         <replace_string> <output_variable>
         <input> [<input>...])

Replace all occurrences of ``<match_string>`` in the ``<input>``
with ``<replace_string>`` and store the result in the ``<output_variable>``.

Regular Expressions
^^^^^^^^^^^^^^^^^^^

.. _`REGEX MATCH`:

.. code-block:: cmake

  string(REGEX MATCH <regular_expression>
         <output_variable> <input> [<input>...])

Match the ``<regular_expression>`` once and store the match in the
``<output_variable>``.
All ``<input>`` arguments are concatenated before matching.

.. _`REGEX MATCHALL`:

.. code-block:: cmake

  string(REGEX MATCHALL <regular_expression>
         <output_variable> <input> [<input>...])

Match the ``<regular_expression>`` as many times as possible and store the
matches in the ``<output_variable>`` as a list.
All ``<input>`` arguments are concatenated before matching.

.. _`REGEX REPLACE`:

.. code-block:: cmake

  string(REGEX REPLACE <regular_expression>
         <replacement_expression> <output_variable>
         <input> [<input>...])

Match the ``<regular_expression>`` as many times as possible and substitute
the ``<replacement_expression>`` for the match in the output.
All ``<input>`` arguments are concatenated before matching.

The ``<replacement_expression>`` may refer to parenthesis-delimited
subexpressions of the match using ``\1``, ``\2``, ..., ``\9``.  Note that
two backslashes (``\\1``) are required in CMake code to get a backslash
through argument parsing.

.. _`Regex Specification`:

Regex Specification
"""""""""""""""""""

The following characters have special meaning in regular expressions:

``^``
  Matches at beginning of input
``$``
  Matches at end of input
``.``
  Matches any single character
``\<char>``
  Matches the single character specified by ``<char>``.  Use this to
  match special regex characters, e.g. ``\.`` for a literal ``.``
  or ``\\`` for a literal backslash ``\``.  Escaping a non-special
  character is unnecessary but allowed, e.g. ``\a`` matches ``a``.
``[ ]``
  Matches any character(s) inside the brackets
``[^ ]``
  Matches any character(s) not inside the brackets
``-``
  Inside brackets, specifies an inclusive range between
  characters on either side e.g. ``[a-f]`` is ``[abcdef]``
  To match a literal ``-`` using brackets, make it the first
  or the last character e.g. ``[+*/-]`` matches basic
  mathematical operators.
``*``
  Matches preceding pattern zero or more times
``+``
  Matches preceding pattern one or more times
``?``
  Matches preceding pattern zero or once only
``|``
  Matches a pattern on either side of the ``|``
``()``
  Saves a matched subexpression, which can be referenced
  in the ``REGEX REPLACE`` operation. Additionally it is saved
  by all regular expression-related commands, including
  e.g. :command:`if(MATCHES)`, in the variables
  :variable:`CMAKE_MATCH_<n>` for ``<n>`` 0..9.

``*``, ``+`` and ``?`` have higher precedence than concatenation.  ``|``
has lower precedence than concatenation.  This means that the regular
expression ``^ab+d$`` matches ``abbd`` but not ``ababd``, and the regular
expression ``^(ab|cd)$`` matches ``ab`` but not ``abd``.

CMake language :ref:`Escape Sequences` such as ``\t``, ``\r``, ``\n``,
and ``\\`` may be used to construct literal tabs, carriage returns,
newlines, and backslashes (respectively) to pass in a regex.  For example:

* The quoted argument ``"[ \t\r\n]"`` specifies a regex that matches
  any single whitespace character.
* The quoted argument ``"[/\\]"`` specifies a regex that matches
  a single forward slash ``/`` or backslash ``\``.
* The quoted argument ``"[A-Za-z0-9_]"`` specifies a regex that matches
  any single "word" character in the C locale.
* The quoted argument ``"\\(\\a\\+b\\)"`` specifies a regex that matches
  the exact string ``(a+b)``.  Each ``\\`` is parsed in a quoted argument
  as just ``\``, so the regex itself is actually ``\(\a\+\b\)``.  This
  can alternatively be specified in a :ref:`bracket argument` without
  having to escape the backslashes, e.g. ``[[\(\a\+\b\)]]``.

Manipulation
^^^^^^^^^^^^

.. _APPEND:

.. code-block:: cmake

  string(APPEND <string_variable> [<input>...])

Append all the ``<input>`` arguments to the string.

.. _PREPEND:

.. code-block:: cmake

  string(PREPEND <string_variable> [<input>...])

Prepend all the ``<input>`` arguments to the string.

.. _CONCAT:

.. code-block:: cmake

  string(CONCAT <output_variable> [<input>...])

Concatenate all the ``<input>`` arguments together and store
the result in the named ``<output_variable>``.

.. _JOIN:

.. code-block:: cmake

  string(JOIN <glue> <output_variable> [<input>...])

Join all the ``<input>`` arguments together using the ``<glue>``
string and store the result in the named ``<output_variable>``.

To join a list's elements, prefer to use the ``JOIN`` operator
from the :command:`list` command.  This allows for the elements to have
special characters like ``;`` in them.

.. _TOLOWER:

.. code-block:: cmake

  string(TOLOWER <string> <output_variable>)

Convert ``<string>`` to lower characters.

.. _TOUPPER:

.. code-block:: cmake

  string(TOUPPER <string> <output_variable>)

Convert ``<string>`` to upper characters.

.. _LENGTH:

.. code-block:: cmake

  string(LENGTH <string> <output_variable>)

Store in an ``<output_variable>`` a given string's length in bytes.
Note that this means if ``<string>`` contains multi-byte characters, the
result stored in ``<output_variable>`` will *not* be the number of characters.

.. _SUBSTRING:

.. code-block:: cmake

  string(SUBSTRING <string> <begin> <length> <output_variable>)

Store in an ``<output_variable>`` a substring of a given ``<string>``.  If
``<length>`` is ``-1`` the remainder of the string starting at ``<begin>``
will be returned.  If ``<string>`` is shorter than ``<length>`` then the
end of the string is used instead.

Both ``<begin>`` and ``<length>`` are counted in bytes, so care must
be exercised if ``<string>`` could contain multi-byte characters.

.. note::
  CMake 3.1 and below reported an error if ``<length>`` pointed past
  the end of ``<string>``.

.. _STRIP:

.. code-block:: cmake

  string(STRIP <string> <output_variable>)

Store in an ``<output_variable>`` a substring of a given ``<string>`` with
leading and trailing spaces removed.

.. _GENEX_STRIP:

.. code-block:: cmake

  string(GENEX_STRIP <string> <output_variable>)

Strip any :manual:`generator expressions <cmake-generator-expressions(7)>`
from the input ``<string>`` and store the result in the ``<output_variable>``.

.. _REPEAT:

.. code-block:: cmake

  string(REPEAT <string> <count> <output_variable>)

Produce the output string as the input ``<string>`` repeated ``<count>`` times.

Comparison
^^^^^^^^^^

.. _COMPARE:

.. code-block:: cmake

  string(COMPARE LESS <string1> <string2> <output_variable>)
  string(COMPARE GREATER <string1> <string2> <output_variable>)
  string(COMPARE EQUAL <string1> <string2> <output_variable>)
  string(COMPARE NOTEQUAL <string1> <string2> <output_variable>)
  string(COMPARE LESS_EQUAL <string1> <string2> <output_variable>)
  string(COMPARE GREATER_EQUAL <string1> <string2> <output_variable>)

Compare the strings and store true or false in the ``<output_variable>``.

.. _`Supported Hash Algorithms`:

Hashing
^^^^^^^

.. _`HASH`:

.. code-block:: cmake

  string(<HASH> <output_variable> <input>)

Compute a cryptographic hash of the ``<input>`` string.
The supported ``<HASH>`` algorithm names are:

``MD5``
  Message-Digest Algorithm 5, RFC 1321.
``SHA1``
  US Secure Hash Algorithm 1, RFC 3174.
``SHA224``
  US Secure Hash Algorithms, RFC 4634.
``SHA256``
  US Secure Hash Algorithms, RFC 4634.
``SHA384``
  US Secure Hash Algorithms, RFC 4634.
``SHA512``
  US Secure Hash Algorithms, RFC 4634.
``SHA3_224``
  Keccak SHA-3.
``SHA3_256``
  Keccak SHA-3.
``SHA3_384``
  Keccak SHA-3.
``SHA3_512``
  Keccak SHA-3.

Generation
^^^^^^^^^^

.. _ASCII:

.. code-block:: cmake

  string(ASCII <number> [<number> ...] <output_variable>)

Convert all numbers into corresponding ASCII characters.

.. _CONFIGURE:

.. code-block:: cmake

  string(CONFIGURE <string> <output_variable>
         [@ONLY] [ESCAPE_QUOTES])

Transform a ``<string>`` like :command:`configure_file` transforms a file.

.. _MAKE_C_IDENTIFIER:

.. code-block:: cmake

  string(MAKE_C_IDENTIFIER <string> <output_variable>)

Convert each non-alphanumeric character in the input ``<string>`` to an
underscore and store the result in the ``<output_variable>``.  If the first
character of the ``<string>`` is a digit, an underscore will also be prepended
to the result.

.. _RANDOM:

.. code-block:: cmake

  string(RANDOM [LENGTH <length>] [ALPHABET <alphabet>]
         [RANDOM_SEED <seed>] <output_variable>)

Return a random string of given ``<length>`` consisting of
characters from the given ``<alphabet>``.  Default length is 5 characters
and default alphabet is all numbers and upper and lower case letters.
If an integer ``RANDOM_SEED`` is given, its value will be used to seed the
random number generator.

.. _TIMESTAMP:

.. code-block:: cmake

  string(TIMESTAMP <output_variable> [<format_string>] [UTC])

Write a string representation of the current date
and/or time to the ``<output_variable>``.

If the command is unable to obtain a timestamp, the ``<output_variable>``
will be set to the empty string ``""``.

The optional ``UTC`` flag requests the current date/time representation to
be in Coordinated Universal Time (UTC) rather than local time.

The optional ``<format_string>`` may contain the following format
specifiers:

::

   %%        A literal percent sign (%).
   %d        The day of the current month (01-31).
   %H        The hour on a 24-hour clock (00-23).
   %I        The hour on a 12-hour clock (01-12).
   %j        The day of the current year (001-366).
   %m        The month of the current year (01-12).
   %b        Abbreviated month name (e.g. Oct).
   %B        Full month name (e.g. October).
   %M        The minute of the current hour (00-59).
   %s        Seconds since midnight (UTC) 1-Jan-1970 (UNIX time).
   %S        The second of the current minute.
             60 represents a leap second. (00-60)
   %U        The week number of the current year (00-53).
   %w        The day of the current week. 0 is Sunday. (0-6)
   %a        Abbreviated weekday name (e.g. Fri).
   %A        Full weekday name (e.g. Friday).
   %y        The last two digits of the current year (00-99)
   %Y        The current year.

Unknown format specifiers will be ignored and copied to the output
as-is.

If no explicit ``<format_string>`` is given, it will default to:

::

   %Y-%m-%dT%H:%M:%S    for local time.
   %Y-%m-%dT%H:%M:%SZ   for UTC.

.. note::

  If the ``SOURCE_DATE_EPOCH`` environment variable is set,
  its value will be used instead of the current time.
  See https://reproducible-builds.org/specs/source-date-epoch/ for details.

.. _UUID:

.. code-block:: cmake

  string(UUID <output_variable> NAMESPACE <namespace> NAME <name>
         TYPE <MD5|SHA1> [UPPER])

Create a universally unique identifier (aka GUID) as per RFC4122
based on the hash of the combined values of ``<namespace>``
(which itself has to be a valid UUID) and ``<name>``.
The hash algorithm can be either ``MD5`` (Version 3 UUID) or
``SHA1`` (Version 5 UUID).
A UUID has the format ``xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx``
where each ``x`` represents a lower case hexadecimal character.
Where required, an uppercase representation can be requested
with the optional ``UPPER`` flag.