CMSC330

A pattern that describes a set of strings

How to define the set?

We write a pattern or a regular expression

Our first pattern

"a"

Describes the set {"a"}

Our second pattern

"hello"

Describes the set {"hello"}

Boring

"hello|hi"

Describes the set {"hello", "hi"}

Boolean Or

"this|that"

Describes the set {"this", "that"}

The or operator's scope extends to start or end

"this|that|the other thing"

or until another |

Describes the set {"this", "that", "the other thing"}

Precedence

"cliff|clyff"

{"cliff","clyff"}

A lot of shared characters

"cl(i|y)ff"

Describes the same set

Quantification

"0|1|2|3|4|5|6|7|8|9"

{"0", "1", "2", "3", "4", "5", "6", "7", "8", "9"}

What about two digit strings?

"(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)"

Cringe

Quantification

"(0|1|2|3|4|5|6|7|8|9)(0|1|2|3|4|5|6|7|8|9)"

"(0|1|2|3|4|5|6|7|8|9){2}"

What about infinite repetition?

Kleene Operator

"(ha)*"

{"", "ha", "haha", "hahaha",...}

Bracket Expressions

"0|1|2|3|4|5|6|7|8|9"

"[0-9]"

"[a-z]"

Any ascii range, can also or

"[a-zA-Z]"

Any lowercase or uppercase letter

Bracket Expressions

Any ascii range, can also or

"[a-zA-Z]"

"[aeiou]"

{"a", "e", "i", "o", "u"}

Can also negate single characters

"[^aeiou]"

Anything except a,e,i,o,u

Other helpful symbols

Note: will need to be escaped to be matched

"1 \+ 2"

	let my_re = Re.compile(Re.Posix.re ("[0-9]+\.[0-9]+")) in
	let did_match = Re.execp my_re "26.19" in
	if did_match then
	print_string "successfully matched"
	else
	ptring_string "unsuccessfully matched"

	let my_re = Re.compile(Re.Posix.re "(([0-9]){3})-([0-9]{3}-[0-9]{4})") in
	let m = Re.exec my_re "123-456-7890" in
	let ac = Re.Group.get m 1 in
	let rest = Re.Group.get m 3 in
	let _ = print_string ("the area code is " ^ ac ) in
	print_string ("\nthe rest is " ^ rest )

	let my_re = Re.compile(Re.Posix.re ("([0-9]+)\.[0-9]+")) in
	let matched = Re.exec my_re "26.19" in
	print_string Re.Group.get matched 1