CS346 Regular Expressions1 Pattern Matching Regular Expression

CS346 Regular Expressions2 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

CS346 Regular Expressions3 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

CS346 Regular Expressions4 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/;

CS346 Regular Expressions5 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

CS346 Regular Expressions6 Pattern Modifiers (Adding flags) Flag(s)Purpose iMakes the match case insensitive /oak/i matches "OAK" and "Oak" gPerforms a global match not just the first igMakes the match case insensitive and global

CS346 Regular Expressions7 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

CS346 Regular Expressions8 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 = ” if ( pos != -1) { alert( ‘First absolute link found at’ + pos +’position.’); } else { alert ( ‘Absolute links not found’); }

CS346 Regular Expressions9 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; }

CS346 Regular Expressions10 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.

CS346 Regular Expressions11 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]

CS346 Regular Expressions12 Character class abbreviations AbbreviationEquiv. PatternMatches \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.

CS346 Regular Expressions13 From Chapter 25 of text - Perl Note the difference of usage of ^ here and in a class

CS346 Regular Expressions14 Quantifiers Quantifiers in braces - Repetitions QuantifierMeaning {n}exactly n repetitions {m,}at least m repetitions {min, max}at least min but max repetitions allowed

CS346 Regular Expressions15 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 but not \n nor l

CS346 Regular Expressions16 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

CS346 Regular Expressions17 Examples  test() See 16-1checkURL.html See 16-2valid .html  search() method in String See 16-3check_phone.html

CS346 Regular Expressions18 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"

CS346 Regular Expressions19 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"]

CS346 Regular Expressions20 match(pattern) example 16-4matchExample.html var 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]

CS346 Regular Expressions21 Parentheses in RE Example: 16-5complexMatchEx.html var str = "I have 118 credits; but I need 120 to graduate"; matches = str.match(/(\d+)([^\d]+)(\d+)/); document.write(matches, " "); 1 st element of matches is the match, 2 nd is the value of $1, 3 rd element $2, 4 th element $3 etc. matches array: 118 credits; but I need 120,118, credits; but I need,120 ______________________ ___ _______________ ___ match with RE $1 $2 $3

CS346 Regular Expressions22 Alternate patterns  Use the alternation operator |  Example: 16-6matchAlternatives.html

CS346 Regular Expressions23 split(parameter) of String splits a string into substrings based on a pattern “:" and /:/ both work Example: 16-7splitEx.html

CS346 Regular Expressions24 Program Structure  Example 16-3check_phone.html  Limitations?  How can you make it more flexible?  Can you generalize it for checking multiple fields

CS346 Regular Expressions25 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 CS346 Regular Expressions26

CS346 Regular Expressions27 Table – Regular Expression Codes  See “Regular Expression Codes.doc”