cs346 regular expressions1 pattern matching regular expression

CS346 Regular Expressions 1

Pattern Matching

Regular Expression


Pattern Matching

JavaScript provides two ways to do pattern matching:

1. Using RegExp objects 2. Using methods on String objects

RE in both ways are the same Same as in Perl


Simple patterns

Two categories of characters in patterns:

a. normal characters (match themselves)

b. metacharacters (can have special meanings in patterns--do not match themselves)

\ | ( ) [ ] { } ^ $ * + ? .

- A metacharacter is treated as a normal character if it is backslashed

- period (.) is a special metacharacter - it matches any character except newline


create RegExp objects

var varname = / reg_ex_pattern / flags

Simplest example: exact match To match occurrence of “our” in a

string containing your, our, sour, four, pour

var toMatch = /our/;


1. Matching in RegExp objects

Tests a string for pattern matches. This method returns a Boolean that indicates whether or not the specified pattern exists within the searched string. This is the most commonly used method for validation. Use test() method of RegExp object

Format: regexp.test( string_to_be_tested ) test() returns a Boolean

var tomatch=/our/;var result = tomatch.test(“pour”); //boolean result

Example: 16-0-checkName.html


Pattern Modifiers (Adding flags)

Flag(s) Purpose

i Makes the match case insensitive/oak/i matches "OAK" and "Oak"

g Performs a global match not just the first

ig Makes the match case insensitive and global


2. Matching in Strings

search() method Returns the position in the specified string of the RE

pattern (position is relative to zero); returns -1 if it fails

var str = "Gluckenheimer"; var position = str.search(/n/); /* position is now 6 */

match() method compares a RE and a string to see whether they

match. replace() method

finds out if a RE matches a string and then replaces a matched string with a new string

search() method

Format: string.search(reg-exp) Searches the string for the first match to the given regular

expression returns an integer that indicates the position in the string

(zero-indexed). If no match is found, the method will return –1.

Similar to the indexOf() method, Example: To find the location of the first absolute link

within a HTML document:: pos = htmlString.search(/^<a href =

”http:\/\/”$/i);if ( pos != -1) { alert( ‘First absolute link found at’ + pos +’position.’);}else { alert ( ‘Absolute links not found’);}


Match() method

match() method Format: string.match( regular_expression ) returns an array of all the matching strings found in the

string given. If no matches are found, then match() returns false.

Example: To check the proper format for a phone number entered by a user, with the form of

(XXX) XXX-XXXX.

function checkPhone( phone ) { phoneRegex = /^$\d\d\d$ \d\d\d-\d\d\d\d$/; if( !phone.match( phoneRegex ) ) { alert( ‘Please enter a valid phone number’ ); return false; } return true;}

replace() method

Format string.replace(reg_exp) Properties: replaces matches to a given regular expression with

some new string. Example: To replace every newline character (\n) with a break

 tag, comment = document.forms[0].comments.value; /* assumes that the HTML form is the first one present in the

document, and it has a field named “comments” */

comment = comment.replace( /\n/g, “ ”); function formatField( fieldValue ) {

return fieldValue = fieldValue. replace(/\n/g, “ ”);}

The function accepts any string as a parameter, and returns the new string with all of the newline characters replaced by tags.


Character classes – [ ]

Sequence of characters in brackets defines a set of characters, any one of which matches

e.g. [abcd]

Dashes used to specify spans of characters in a class

e.g. [a-z]

A caret at the left end of a class definition means the opposite

e.g. [^0-9]


Character class abbreviations

Abbreviation Equiv. Pattern Matches

\d [0-9] a digit

\D [^0-9] not a digit

\w [A-Za-z_0-9] a word char.

\W [^A-Za-z_0-9] not a word char.

\s [ \r\t\n\f] a whitespace char.

\S [^ \r\t\n\f] not a whitespace

char.


From Chapter 25 of text - Perl

Symbol Matches Symbol Matches ^ Beginning of line \d Digit (i.e., 0 to 9) $ End of line \D Nondigit \b Word boundary \s Whitespace \B Nonword boundary \S Nonwhitespace \w Word (alphanumeric)

character \n Newline

\W Nonword character \t Tab Fig. 25.9 Some of Perl’s metacharacters.

Note the difference of usage of ^ here and in a class


Quantifiers

Quantifiers in braces - Repetitions

Quantifier Meaning{n} exactly n repetitions{m,} at least m repetitions{min, max} at least min but max

repetitions allowed


Some other common Quantifiers

* zero or more repetitions

e.g., \d* means zero or more digits + one or more repetitions e.g., \d+ means one or more digits ? zero or one e.g., \d? means zero or one digit . exactly one character except

newline character e.g., /.l/ matches al or @l but not \n

nor l


Anchors

The pattern can be forced to match only at the left end with ^; at the end with $

e.g., /^Lee/ matches "Lee Ann" but not "Mary Lee Ann"

/Lee Ann$/ matches "Mary Lee Ann", but not "Mary Lee Ann is nice“

The anchor operators (^ and $) do not match characters in the string--they match positions, at the beginning or end


Examples

test() See 16-1checkURL.html See 16-2validEmail.html

search() method in String See 16-3check_phone.html


replace method()

replace(RE_pattern, string)

Finds a substring that matches the pattern replaces it with the string g modifier applicable

var str = "Some rabbits are rabid"; str.replace(/rab/g, "tim");

str is now "Some timbits are timid“ Matched substrings stored in $1, $2, etc $1 and $2 are both set to "rab"


match(pattern)

Most general pattern-matching method Returns an array of results of the pattern-matching

operation

With the g modifier, returns an array of the substrings that matched

Without the g modifier, first element of the returned array has the matched substring, the other elements have the values of $1, … obtained by parenthesized parts of pattern

var str = "My 3 kings beat your 2 aces"; var matches = str.match(/[ab]/g);

- matches is set to ["b", "a", "a"]


match(pattern) example

16-4matchExample.htmlvar str = “Having a take-home exam that

takes 3 hours to complete is better than a 1-hour in-class exam”;

var matches = str.match( /\d/g );

matches is set to [3, 1]

Parentheses in RE

Example: 16-5complexMatchEx.htmlvar str = "I have 118 credits; but I need 120 to graduate";

matches = str.match(/(\d+)([^\d]+)(\d+)/);document.write(matches, " ");

1st element of matches is the match, 2nd is the value of $1, 3rd element $2, 4th element $3 etc.

matches array:118 credits; but I need 120,118, credits; but I need ,120______________________ ___ _______________ ___ match with RE $1 $2 $3


Alternate patterns

Use the alternation operator | Example: 16-6matchAlternatives.html


split(parameter) of String

splits a string into substrings based on a pattern

“:" and /:/ both work

Example: 16-7splitEx.html


Program Structure

Example 16-3check_phone.html Limitations? How can you make it more flexible? Can you generalize it for checking

multiple fields


Uniform Program Structure for multiple tests

regex_name.test( string_to_be_tested ) to test each field

if test() returns false, compile an error message

See 16-8Structure.html

Examples of curly braces { }

16-9-curly_braces.html



Table – Regular Expression Codes

See “Regular Expression Codes.doc”

cs346 regular expressions1 pattern matching regular expression

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