Introducing flex-2

Lecture 2
Introducing Flex (2)

2.1Regular Expressions

The patterns at the heart of every flex scanner use a rich regular expression language.
A regular expression is a pattern description using a metalanguage, a language that you use to describe what you want the pattern to match. The metalanguage uses standard text characters, some of which represent themselves and others of which represent patterns. All characters other than the ones listed below, including all letters and digits, match themselves.
The characters with special meaning in regular expressions are:

. Matches any single character except the newline character (\n).
* Matches zero or more copies of the preceding expression.
+ Matches one or more occurrences of the preceding regular expression
? Matches zero or one occurrence of the preceding regular expression
| The alternation operator
[ ] A character class that matches any character within the brackets
" " Anything within the quotation marks is treated literally
( ) Groups a series of regular expressions together into a new regular expression.
{ } If the braces contain one or two numbers, indicate the minimum and maximum
number of times the previous pattern can match. If the braces contain a name, they refer to a named pattern by that name.
^ Matches the beginning of a line as the first character of a regular expression
$ Matches the end of a line as the last character of a regular expression
/ Trailing context, which means to match the regular expression preceding the slash but only if followed by the regular expression after the slash a/b
\ Used to escape metacharacters and as part of the usual C escape sequences; for
Example,\n is a newline character, while\* is a literal asterisk.