html2 Flex - a scanner generator: 7. Patterns

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7. Patterns

The patterns in the input are written using an extended set of regular expressions. These are:

‘x’

match the character ‘x’

‘.’

any character (byte) except newline

‘[xyz]’

a "character class"; in this case, the pattern matches either an ‘x’, a ‘y’, or a ‘z’

‘[abj-oZ]’

a "character class" with a range in it; matches an ‘a’, a ‘b’, any letter from ‘j’ through ‘o’, or a ‘Z’

‘[^A-Z]’

a "negated character class", i.e., any character but those in the class. In this case, any character EXCEPT an uppercase letter.

‘[^A-Z\n]’

any character EXCEPT an uppercase letter or a newline

‘r*’

zero or more r’s, where r is any regular expression

‘r+’

one or more r’s

‘r?’

zero or one r’s (that is, "an optional r")

‘r{2,5}’

anywhere from two to five r’s

‘r{2,}’

two or more r’s

‘r{4}’

exactly 4 r’s

‘{name}’

the expansion of the "name" definition (see above)

‘"[xyz]\"foo"’

the literal string: ‘[xyz]"foo’

‘\x’

if x is an ‘a’, ‘b’, ‘f’, ‘n’, ‘r’, ‘t’, or ‘v’, then the ANSI-C interpretation of \x. Otherwise, a literal ‘x’ (used to escape operators such as ‘*’)

‘\0’

a NUL character (ASCII code 0)

‘\123’

the character with octal value 123

‘\x2a’

the character with hexadecimal value 2a

‘(r)’

match an r; parentheses are used to override precedence (see below)

‘rs’

the regular expression r followed by the regular expression s; called "concatenation"

‘r|s’

either an r or an s

‘r/s’

an r but only if it is followed by an s. The text matched by s is included when determining whether this rule is the longest match, but is then returned to the input before the action is executed. So the action only sees the text matched by r. This type of pattern is called trailing context. (There are some combinations of ‘r/s’ that flex cannot match correctly; see notes in the Deficiencies / Bugs section below regarding "dangerous trailing context".)

‘^r’

an r, but only at the beginning of a line (i.e., which just starting to scan, or right after a newline has been scanned).

‘r$’

an r, but only at the end of a line (i.e., just before a newline). Equivalent to "r/\n".

Note that flex’s notion of "newline" is exactly whatever the C compiler used to compile flex interprets ’\n’ as; in particular, on some DOS systems you must either filter out \r’s in the input yourself, or explicitly use r/\r\n for "r$".

‘<s>r’

an r, but only in start condition s (see below for discussion of start conditions) <s1,s2,s3>r same, but in any of start conditions s1, s2, or s3

‘<*>r’

an r in any start condition, even an exclusive one.

‘<<EOF>>’

an end-of-file <s1,s2><<EOF>> an end-of-file when in start condition s1 or s2

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:

- A negated character class such as the example "[^A-Z]" above will match a newline unless "\n" (or an equivalent escape sequence) is one of the characters explicitly present in the negated character class (e.g., "[^A-Z\n]"). This is unlike how many other regular expression tools treat negated character classes, but unfortunately the inconsistency is historically entrenched. Matching newlines means that a pattern like [^"]* can match the entire input unless there’s another quote in the input.
- A rule can have at most one instance of trailing context (the ’/’ operator or the ’$’ operator). The start condition, ’^’, and "<<EOF>>" patterns can only occur at the beginning of a pattern, and, as well as with ’/’ and ’$’, cannot be grouped inside parentheses. A ’^’ which does not occur at the beginning of a rule or a ’$’ which does not occur at the end of a rule loses its special properties and is treated as a normal character.
The following are illegal:
foo/bar$ <sc1>foo<sc2>bar
Note 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|^bar
If 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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This document was generated on December 29, 2020 using texi2html 1.82.