The patterns in the input are written using an extended set of regular expressions. These are:
2a
flex
cannot match correctly; see notes in the Deficiencies / Bugs section
below regarding "dangerous trailing context".)
Note that inside of a character class, all regular expression operators lose their special meaning except escape ('\') and the character class operators, '-', ']', and, at the beginning of the class, '^'.
The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. Those grouped together have equal precedence. For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than concatenation, and concatenation higher than alternation ('|'). This pattern therefore matches either the string "foo" or the string "ba" followed by zero-or-more r's. To match "foo" or zero-or-more "bar"'s, use:
foo|(bar)*
and to match zero-or-more "foo"'s-or-"bar"'s:
(foo|bar)*
In addition to characters and ranges of characters, character classes can also contain character class expressions. These are expressions enclosed inside `[': and `:'] delimiters (which themselves must appear between the '[' and ']' of the character class; other elements may occur inside the character class, too). The valid expressions are:
[:alnum:] [:alpha:] [:blank:] [:cntrl:] [:digit:] [:graph:] [:lower:] [:print:] [:punct:] [:space:] [:upper:] [:xdigit:]
These expressions all designate a set of characters equivalent to the corresponding standard C `isXXX' function. For example, `[:alnum:]' designates those characters for which `isalnum()' returns true - i.e., any alphabetic or numeric. Some systems don't provide `isblank()', so flex defines `[:blank:]' as a blank or a tab.
For example, the following character classes are all equivalent:
[[:alnum:]] [[:alpha:][:digit:] [[:alpha:]0-9] [a-zA-Z0-9]
If your scanner is case-insensitive (the `-i' flag), then `[:upper:]' and `[:lower:]' are equivalent to `[:alpha:]'.
Some notes on patterns:
foo/bar$ <sc1>foo<sc2>barNote that the first of these, can be written "foo/bar\n". The following will result in '$' or '^' being treated as a normal character:
foo|(bar$) foo|^barIf what's wanted is a "foo" or a bar-followed-by-a-newline, the following could be used (the special '|' action is explained below):
foo | bar$ /* action goes here */A similar trick will work for matching a foo or a bar-at-the-beginning-of-a-line.
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