Identifiers
An identifier is a series of characters that are not contained in a comment or a constant.
Except for P, PIC, and PICTURE, identifiers must be preceded and
followed by a delimiter. (P, PIC, and PICTURE identifiers can be followed
by a character string.) The first character of an identifier must
be an alphabetic or extralingual character. If the identifier names
an INTERNAL symbol, it can also use the break (_) character as its
first character. Other characters, if any, can be alphabetic, extralingual,
digit, or the break (_) character. External user names must not start
with IBM
, PLI
, CEE
, _IBM
, _PLI
,
and _CEE
.
Identifiers can be PL/I keywords or programmer-defined names. Because PL/I can determine from the context if an identifier is a keyword, you can use any identifier as a programmer-defined name. There are no reserved words in PL/I. However, using some keywords, for example, IF or THEN, as variable names might make a program needlessly hard to understand.