Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming lex: Lexical Analyzer Generator
Specifying Alternation and Grouping
The vertical bar (I) operator indicates alternation. For example,
(ab I cd)
matches either ab or cd. Note that parentheses are used for grouping, although they are
not necessary at the outside level. For example
ab l cd
would have sufficed in the preceding example. Parentheses should be used for more
complex expressions, such as
(ab I cd + )?(ef) *
which matches such strings as abefef, efefef, cdef, and cddd, but not abc, abed, or
abcdef.
Specifying Context Sensitivity
lex recognizes a small amount of surrounding context. The two simplest operators for
this are the caret (") and the dollar sign ($). If the first character of an expression is a
caret, then the expression is only matched at the beginning of a line (after a newline
character or at the beginning of the input stream). This can never conflict with the
other meaning of the caret, complem entation of character classes, since
complementation applies only within brackets. If the very last character is dollar sign,
the expression is only matched at the end of a line (when immediately followed by a
newline). The latter operator is a special case of the slash (/) operator, which indicates
trailing context. The expression
ab/cd
matches the string ab, but only if followed by cd. Thus
ab$
is the same as
ab/ \n
Left context is handled in lex by specifying start conditions as explained in the section
"Specifying Left Context Sensitivity." If a rule is only to be executed when the lex
automaton interpreter is in start condition x, the rule should be enclosed in angle
brackets:
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