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An identifier is the name of an element in the code. It can contain letters, digits and underscores (_), and is case sensitive (FOO is different from foo). The language imposes the following restrictions on identifier names: They cannot start with a digit; They cannot start with a symbol, unless it is a keyword;
A system that is non-case-preserving is necessarily also case-insensitive. This applies, for example, to Identifiers (column and table names) in some relational databases (for example DB2, Interbase/Firebird, Oracle and Snowflake [1]), unless the identifier is specified within double quotation marks (in which case the identifier becomes case-sensitive).
Some programming languages are case-sensitive for their identifiers (C, C++, Java, C#, Verilog, [2] Ruby, [3] Python and Swift).Others are case-insensitive (i.e., not case-sensitive), such as ABAP, Ada, most BASICs (an exception being BBC BASIC), Common Lisp, Fortran, SQL (for the syntax, and for some vendor implementations, e.g. Microsoft SQL Server, the data itself) [NB 2] Pascal, Rexx and ...
Identifier length rules are routinely contested in practice, and subject to much debate academically. Some considerations: shorter identifiers may be preferred as more expedient, because they are easier to type (although many IDEs and text-editors provide text-completion, which mitigates this)
A global identifier is declared outside of functions and is available throughout the program. A local identifier is declared within a specific function and only available within that function. [1] For implementations of programming languages that are using a compiler, identifiers are often only compile time entities.
All unquoted snake_case identifiers are actually internally represented as SCREAMING_SNAKE_CASE identifiers. Prolog, for both atoms (predicate names, function names, and constants) and variables [20] Python, for variable names, function names, method names, and module or package (i.e. file) names [3] PHP uses SCREAMING_SNAKE_CASE for class ...
C and C++ are notable in this respect: C99 reserves identifiers that start with two underscores or an underscore followed by an uppercase letter, and further reserves identifiers that start with a single underscore (in the ordinary and tag spaces) for use in file scope; [1] with C++03 further reserves identifiers that contain a double ...
MPEG-1 Layer 3 file without an ID3 tag or with an ID3v1 tag (which is appended at the end of the file) 49 44 33: ID3: 0 mp3 MP3 file with an ID3v2 container 42 4D: BM: 0 bmp dib BMP file, a bitmap format used mostly in the Windows world 43 44 30 30 31: CD001: 0x8001 0x8801 0x9001 iso ISO9660 CD/DVD image file [40] 43 44 30 30 31: CD001: 0x5EAC9 ...