2.1 – Lexical Conventions

Identifiers in Lua can be any string of letters, digits, and underscores, not beginning with a digit. This coincides with the definition of identifiers in most languages. (The definition of letter depends on the current locale: any character considered alphabetic by the current locale can be used in an identifier.)

The following keywords are reserved and cannot be used as identifiers:

  1.        and       break     do        else      elseif
  2.        end       false     for       function  if
  3.        in        local     nil       not       or
  4.        repeat    return    then      true      until     while

Lua is a case-sensitive language: and is a reserved word, but And and AND are two different, valid identifiers. As a convention, identifiers starting with an underscore followed by uppercase letters (such as _VERSION) are reserved for internal variables used by Lua.

The following strings denote other tokens:

  1.        +     -     *     /     ^     =
  2.        ~=    <=    >=    <     >     ==
  3.        (     )     {     }     [     ]
  4.        ;     :     ,     .     ..    ...

Literal strings can be delimited by matching single or double quotes, and can contain the following C-like escape sequences:

  • \a —- bell
  • \b —- backspace
  • \f —- form feed
  • \n —- newline
  • \r —- carriage return
  • \t —- horizontal tab
  • \v —- vertical tab
  • \\ —- backslash
  • \" —- quotation mark
  • \' —- apostrophe
  • \[ —- left square bracket
  • \] —- right square bracket

Moreover, a `\newline´ (that is, a backslash followed by a real newline) results in a newline in the string. A character in a string may also be specified by its numerical value using the escape sequence `\ddd´, where ddd is a sequence of up to three decimal digits. Strings in Lua may contain any 8-bit value, including embedded zeros, which can be specified as `\0´.

Literal strings can also be delimited by matching double square brackets [[ · · · ]]. Literals in this bracketed form may run for several lines, may contain nested [[ · · · ]] pairs, and do not interpret any escape sequences. For convenience, when the opening `[[´ is immediately followed by a newline, the newline is not included in the string. As an example, in a system using ASCII (in which `a´ is coded as 97, newline is coded as 10, and `1´ is coded as 49), the four literals below denote the same string:

  1.       (1)   "alo\n123\""
  2.       (2)   '\97lo\10\04923"'
  3.       (3)   [[alo
  4.             123"]]
  5.       (4)   [[
  6.             alo
  7.             123"]]

Numerical constants may be written with an optional decimal part and an optional decimal exponent. Examples of valid numerical constants are

  1.        3     3.0     3.1416  314.16e-2   0.31416E1

Comments start anywhere outside a string with a double hyphen (--). If the text immediately after -- is different from [[, the comment is a short comment, which runs until the end of the line. Otherwise, it is a long comment, which runs until the corresponding ]]. Long comments may run for several lines and may contain nested [[ · · · ]] pairs.

For convenience, the first line of a chunk is skipped if it starts with #. This facility allows the use of Lua as a script interpreter in Unix systems (see 6).