x

match the character `x'

.

any character (byte) except newline

[xyz]

a "character class"; in this example, 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 example, 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

"[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

rs

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

r|s

either an r or an s

^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).

<s>r

an r, but only in start condition s.

<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

The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom.