DIG 3563: Lecture 2a: Regular Expressions Michael Moshell University of Central Florida

transcript

DIG 3563: Lecture 2a:

Regular Expressions

Michael MoshellUniversity of Central Florida

Information Management

If you don’t know how toDo something,

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Tell meOr

Come see me.

DO NOT BEA RABBIT!

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Regular Expressions• A "grammar" for validating input useful for many kinds of pattern recognition

The basic built-in Boolean function in PHP is called 'preg_match'.

It takes two or three arguments:

the pattern, like "cat"the test string, like "catastrophe"

and an (optional) array variable, which we can ignore for now

It returns TRUE if the pattern matches the test string.

POSIX Regular Expressions

Always begin with "/ and end with /" (for today's lesson)

$instring = "catastrophe";

if (preg_match("/cat/",$instring)){

print "I found a cat!";}else{

print "No cat here.";}

Regular Expressions

$instring = "catastrophe";

if (preg_match("/cat/",$instring)){

print "I found a cat!";}else{

print "No cat here.";}

I found a cat!

PRACTICE 1:

"/cat/" that is the regular expression

Make up a Regular Expression to recognizeNot the word cat, but rather the word dog.

Write it on your paper, now.

PRACTICE 1:

Yes, I mean YOU. Where is your paper and pencil?

(You can use your laptop if that’s what you have…)

PRACTICE 1:

Answer: "/dog/"

Yep, it’s that simple. But I gotta get you STARTED.

Regular Expressions

Wild cards: period . matches any single character

$instring = "cotastrophe";

if (preg_match("/c.t/",$instring)){

print "I found a c.t!";}else{

print "No c.t here.";}

Regular Expressions

Wild cards: period . matches any single character

if (preg_match("/c.t/",$instring)){

print "I found matching string!";}else{

print "No c.t here.";}

I found a matching string!

Regular Expressions

Wild cards: a* matches any number of a characters (or the "null character"!)

$instring = "caaaatastrophe";

if (preg_match("/ca*t/",$instring)){

print "I found a match!";}else{

print "No ca*t here.";}

I found a match!

Regular Expressions

Wild cards: .* matches any string of characters (or the "null character"!)

if (preg_match("/c.*t/",$instring)){

print "I found a c.*t!";}else{

print "No c.*t here.";}

I found a c.*t!

Regular Expressions

$instring = "cflippingmonstroustastrophe";

Regular Expressions

$instring = "cflippingmonstroustastrophe";

I found a c.*t!

PRACTICE 2:

"/c.t/" that is a model RE for you"/c.*t/" that is a model RE for you"/ca*t/" that is a model RE for you

Make up a Regular Expression to recognize

Rob or Rb or Roob or Rooob, etc.

But to REJECT Reb and Rab and Rats and Mike ….

PRACTICE 2:

"/c.t/" that is a model RE for you"/c.*t/" that is a model RE for you"/ca*t/" that is a model RE for you

Answer:

”/Ro*b/”

Quantification

Multiple copies of something:

a+ means ONE OR MORE a’sExample: "/fa+ther/" matches father, faather, faaather, etc.

a* means ZERO OR MORE a’sExample: "/fa*ther/" matches fther, father, faather, etc.

a? means ZERO OR ONE aExample: "/flavou?r/" will match flavor AND flavour.

a{33} means 33 instances of a

Quantification Example

How to recognize “Rob” or “Robb”?

How to recognize “Rob” or “Robb”? ”/Robb?/"

How to recognize “Rob” or “Robb”? Another way:”/Rob{1,2}/"

Escaping

Backslash means "don't interpret this:"

\. is just a dot\* is just an asterisk.

The concept:

$t="/a{3}\.b{1,4}/";$s= "aaa.bbb"; this would or would not be accepted?

preg_match($t,$s) – true or false?

The concept:

$t="/a{3}\.b{1,4}/";$s= "aaa.bbb"; this would or would not be accepted?

TRUE, because $s matches the pattern string $t.

three a, one dot, and between one and four b characters.

The concept:

$t="/a{3}\.b{1,4}/";$s= "aaa.bbbbb"; this would or would not be accepted?

The concept:

Perhaps surprisingly, TRUE: because $scontains three a and 4 b.

The concept:

If you have $1.00 and I asked you “do you have 75 cents?” the answer would be YES.

The concept:

If you wanted an EXACT match, I'll show you howIn a bit.

Grouping

Multiple copies of something:

(abc)+ means ONE OR MORE string abc’s(abc)* means ZERO OR MORE string abc’s

like abcabcabcSETS:[0-9] matches any single integer character[A-Z] matches any uppercase letter[AZ] matches A or Z[AZ]? (i.e. 0 or 1 of the previous) matches null, A or Z

Starting and Ending

preg_match("/cat/","abunchofcats") is TRUEbutpreg_match("/^cat/","abunchofcats") is FALSE

because ^ means the RE must match the first letter.

preg_match("/cats$/","abunchofcats") is TRUEbutpreg_match("/cats$/","mycatsarelazy") is FALSE

So, ^ marks the head and $ marks the tail.

Exact Matching with ^ and $

$t="/^a{3}\.b{1,4}$/";$s= "aaa.bbbbb"; this would or would not be accepted?

FALSE, because the ending $ in the pattern says "no more input is acceptable" but more stuff comes.

This would also reject$s="aaa.bbbbAndMoreText";

313131

Alternatives - the 'or' mark |

$t="/flav(o|ou)r/";

This will match 'flavor' and 'flavour'.

And (yes!) there are often more than one way to do things; for instance our good old ? Mark.

"/flavou?r/"

Sets - Examples

[A-E]{3} matches AAA, ABA, ADD, ... EEE

[PQX]{2,4} matches PP, PQ, PX ... up to XXXX

[A-Za-z]+ matches any alphabetic string with 1 or more characters

[A-Z][a-z]* matches any alpha string with first letter capitalized.

[a-z0-9]+ matches any string of lowercase letters and numerals

Practice in class

Write a RE that recognizes any string that begins with"sale".

Here's an example for you to look at, help you remember

From now on, the RE is just ^cat. You don't need to write the other stuff (preg_match, "/, etc.)

Practice

1) Write a RE that recognizes any string that begins with"sale".

Answer: ^sale

Practice

Answer: ^sale

2) Write a RE that recognizes a string that begins with"smith" and a two digit integer, like smith23 or smith99.

Here's an example from your recent past: a{3}\.b{1,4}

Practice

Answer: ^sale

2) Write a RE that recognizes a string that begins with"smith" and a two digit integer, like smith23 or smith99.

Answer: ^smith[0-9]{2}

3) Write a RE that recognizes Social Security numbers in the form like

123-45-6789

Helpers from the recent past: ^smith[0-9]{2}

a{3}\.b{1,4}

383838

123-45-6789

Answer:

[0-9]{3}\-[0-9]{2}\-[0-9]{4}

39393939

123-45-6789

Answer:

[0-9]{3}\-[0-9]{2}\-[0-9]{4}

NOTE: That's a conservative answer. It turns out that the dash character is not a special symbol outside sets, and so you could also write

[0-9]{3}-[0-9]{2}-[0-9]{4}

But I don't like to remember stuff, so I use \ a lot.

How to study this stuff?

Practice making up RE for problems like these:

• The UCF NID• French telephone numbers like (+33 5 23 46 22 91)• Dollars and cents, like $942.73• A field that may contain only lowercase strings with

exactly ONE vowel.

How do you know if they're good? If you know PHPYou can test them. Otherwise, check out each others' work.

(OR come see me in office hours!)(Or by appointment!)407 694 6763

DIG 3563: Lecture 2a: Regular Expressions Michael Moshell University of Central Florida

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