Regular Expressions Syntax

A regular expression is like a complex "wildcard". In other words, it's a string that describes or matches other strings, according to certain syntax rules.

Iotellect uses Java syntax for regular expression processing. See links below for more info.

Summary of Regular Expression Constructs

Construct

Matches

Characters

x

The character x

\

The backslash character

n

The character with octal value 0n (0 <= n <= 7)

nn

The character with octal value 0nn (0 <= n <= 7)

mnn

The character with octal value 0mnn (0 <= m <= 3, 0 <= n <= 7)

xhh

The character with hexadecimal value 0xhh

\uhhhh

The character with hexadecimal value 0xhhhh

t

The tab character ('\u0009')

n

The newline (line feed) character ('\u000A')

r

The carriage-return character ('\u000D')

f

The form-feed character ('\u000C')

a

The alert (bell) character ('\u0007')

e

The escape character ('\u001B')

cx

The control character corresponding to x

Character classes

[abc]

a, b, or c (simple class)

[^abc]

Any character except a, b, or c (negation)

[a-zA-Z]

a through z or A through Z, inclusive (range)

[a-d[m-p]]

a through d, or m through p: [a-dm-p] (union)

[a-z&&[def]]

d, e, or f (intersection)

[a-z&&[^bc]]

a through z, except for b and c: [ad-z] (subtraction)

[a-z&&[^m-p]]

a through z, and not m through p: [a-lq-z](subtraction)

Predefined character classes

.

Any character (may or may not match line terminators)

d

A digit: [0-9]

D

A non-digit: [^0-9]

s

A whitespace character: [ tnx0Bfr]

S

A non-whitespace character: [^s]

w

A word character: [a-zA-Z_0-9]

W

A non-word character: [^w]

POSIX character classes (US-ASCII only)

p{Lower}

A lower-case alphabetic character: [a-z]

p{Upper}

An upper-case alphabetic character:[A-Z]

p{ASCII}

All ASCII:[x00-x7F]

p{Alpha}

An alphabetic character:[p{Lower}p{Upper}]

p{Digit}

A decimal digit: [0-9]

p{Alnum}

An alphanumeric character:[p{Alpha}p{Digit}]

p{Punct}

Punctuation: One of !"#$%&'()*+,-./:;<=>?@[]^_`{|}~

p{Graph}

A visible character: [p{Alnum}p{Punct}]

p{Print}

A printable character: [p{Graph}x20]

p{Blank}

A space or a tab: [ t]

p{Cntrl}

A control character: [x00-x1Fx7F]

p{XDigit}

A hexadecimal digit: [0-9a-fA-F]

p{Space}

A whitespace character: [ tnx0Bfr]

Boundary matchers

^

The beginning of a line

$

The end of a line

b

A word boundary

B

A non-word boundary

A

The beginning of the input

G

The end of the previous match

Z

The end of the input but for the final terminator, if any

z

The end of the input

Greedy quantifiers

X?

X, once or not at all

X*

X, zero or more times

X+

X, one or more times

X{n}

X, exactly n times

X{n,}

X, at least n times

X{n,m}

X, at least n but not more than m times

Reluctant quantifiers

X??

X, once or not at all

X*?

X, zero or more times

X+?

X, one or more times

X{n}?

X, exactly n times

X{n,}?

X, at least n times

X{n,m}?

X, at least n but not more than m times

Possessive quantifiers

X?+

X, once or not at all

X*+

X, zero or more times

X++

X, one or more times

X{n}+

X, exactly n times

X{n,}+

X, at least n times

X{n,m}+

X, at least n but not more than m times

Logical operators

XY

X followed by Y

X|Y

Either X or Y

(X)

X, as a capturing group

Back references

n

Whatever the nth capturing group matched

Quotation

Nothing, but quotes the following character

Q

Nothing, but quotes all characters until E

E

Nothing, but ends quoting started by Q

Backslashes, Escapes, and Quoting

The backslash character ('') serves to introduce escaped constructs, as defined in the table above, as well as to quote characters that otherwise would be interpreted as unescaped constructs. Thus the expression \ matches a single backslash and { matches a left brace.

It is an error to use a backslash prior to any alphabetic character that does not denote an escaped construct; these are reserved for future extensions to the regular-expression language. A backslash may be used prior to a non-alphabetic character regardless of whether that character is part of an unescaped construct.

Character Classes

Character classes may appear within other character classes, and may be composed by the union operator (implicit) and the intersection operator (&&). The union operator denotes a class that contains every character that is in at least one of its operand classes. The intersection operator denotes a class that contains every character that is in both of its operand classes.

The precedence of character-class operators is as follows, from highest to lowest:

1

Literal escape

x

2

Grouping

[...]

3

Range

a-z

4

Union

[a-e][i-u]

5

Intersection

[a-z&&[aeiou]]

Note that a different set of metacharacters are in effect inside a character class than outside a character class. For instance, the regular expression . loses its special meaning inside a character class, while the expression - becomes a range forming metacharacter.

Line Terminators

A line terminator is a one- or two-character sequence that marks the end of a line of the input character sequence. The following are recognized as line terminators:

  • A newline (line feed) character ('n'),
  • A carriage-return character followed immediately by a newline character ("rn"),
  • A standalone carriage-return character ('r'),
  • A next-line character ('\u0085'),
  • A line-separator character ('\u2028'), or
  • A paragraph-separator character ('\u2029).

Groups and Capturing

Capturing groups are numbered by counting their opening parentheses from left to right. In the expression ((A)(B(C))), for example, there are four such groups:

1

((A)(B(C)))

2

(A)

3

(B(C))

4

(C)

Group zero always stands for the entire expression.

Capturing groups are so named because, during a match, each subsequence of the input sequence that matches such a group is saved. The captured subsequence may be used later in the expression, via a back reference, and may also be retrieved from the matcher once the match operation is complete.

The captured input associated with a group is always the subsequence that the group most recently matched. If a group is evaluated a second time because of quantification then its previously-captured value, if any, will be retained if the second evaluation fails. Matching the string aba against the expression (a(b)?)+, for example, leaves group two set to b. All captured input is discarded at the beginning of each match.

Groups beginning with (? are pure, non-capturing groups that do not capture text and do not count towards the group total.

More Info on Regular Expressions

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