The identifier is a unique name, which identifies an object.
An identifier is used as an object reference. An identifier can contain a sequence of letters, digits, underscores (_) and dollar signs ($). The first character of an identifier can only be a letter or an underscore (Example 1). Identifiers are case sensitive.
Escaped identifiers (Example 2) start with backslash character (\) and end with white space (i.e. space, tab, new line). Escaped identifiers can contain any printable characters. Leading backslash character and white space at the end are not considered as part of an identifier, thus identifiers \enable and enable are identical.
wire \+^_^+*+*<-> ;
reg \!clk ;
White spaces between an escaped identifier and the semicolon that follows it are required, because otherwise, the semicolon will be taken as a part of an identifier.
reg \clk ;
defines \clk identifier, but
defines \clk; identifier (and there will be also an error message because a variable declaration should be ended by semicolon).
Identifiers are case sensitive
\clk and clk are the same identifiers
Escaped identifiers should be followed by at least a white space character