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The {n,m} Syntax

The

*

operator that was described a little earlier in this appendix effectively means “Match a minimum

of zero occurrences and a maximum occurrence, which is unbounded.” Similarly, the

+

quantifier means

“Match a minimum of one occurrence and a maximum occurrence, which is unbounded.”

Using curly braces and numbers inside them allows the developer to create occurrence quantifiers that

cannot be specified when using the

?

,

*

, or

+

quantifiers.

The following subsections look at three variants that use the curly-brace syntax. First, look at the syntax

that specifies “Match zero or up to [a specified number] of occurrences.”

{0,m}

The

{0,m}

syntax allows you to specify that a minimum of zero occurrences can be matched (specified

by the first numeric digit after the opening curly brace) and that a maximum of

m

occurrences can be

matched (specified by the second numeric digit, which is separated from the minimum occurrence indi-

cator by a comma and which precedes the closing curly brace).

To match a minimum of zero occurrences and a maximum of one occurrence, you would use the pattern

{0,1}

which has the same meaning as the

?

quantifier.

To specify matching of a minimum of zero occurrences and a maximum of three occurrences, you would

use the pattern

{0,3}

which you couldn’t express using the

?

,

*

, or

+

quantifiers.

Suppose that you want to specify that you want to match the sequence of characters

ABC

followed by a

minimum of zero numeric digits or a maximum of two numeric digits.

You can semiformally express that as the following problem definition:

Match an uppercase

A

. If there is a match, attempt to match an uppercase

B

. If there is a match, attempt

to match an uppercase

C

. If all three uppercase characters match, attempt to match a minimum of zero

or a maximum of two numeric digits.

The following pattern does what you need:

ABC[0-9]{0,2}

The

ABC

simply matches a sequence of the corresponding literal characters. The

[0-9]

indicates that a

numeric digit is to be matched, and the

{0,2}

is a quantifier that indicates a minimum of zero occur-

rences of the preceding chunk (which is

[0-9]

, representing a numeric digit) and a maximum of two

occurrences of the preceding chunk is to be matched.

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Appendix A: Simple Regular Expressions

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