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# Regular expressions

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### Regular expressions

1. 1. RegularExpressions
2. 2. Agenda What are regular expressions Need for regular expressions Basic rules Practical examples Regular expression groups search() match() replace()
3. 3. What are Regular expressionsA regular expression is an object that describes a pattern oftext characters.There are two ways of defining a regular expression :var regex=new RegExp(pattern,modifiers); orvar regex=/pattern/modifiers;Example of pattern : ^[a-zA-Z0-9]+\$Modifiers :g - global, i - ignore case, m - multiline
4. 4. Without regular expressions ...The following javascript function tests if a string contains onlyalpha-numeric characters :function bTestOnlyAlphaNum(strToTest) {if (strToTest.length == 0) return false; for (var i=0; i < strToTest.length; i++) { var testChar = strToTest.charCodeAt(i); if ((testChar < 48 || testChar > 57) && (testChar < 65 || testChar > 90) && (testChar < 97 || testChar > 122)) return false; } return true;}
5. 5. Magic of regular expressionsBy using regular expressions, the same functionality as shownin the previous slide can be achieved as :function bTestOnlyAlphaNum(strToTest) { return (strToTest.match(/^[a-zA-Z0-9]+\$/) != null);}We will get into details of this regular expression later. Let usfirst walk through the basic rules of regular expressions.
6. 6. Basic Rules . Matches any one character, except for line breaks. * Matches 0 or more of the preceding character. + Matches 1 or more of the preceding character. ? Preceding character is optional. Matches 0 or 1 occurrence. d Matches any single digit (opposite: D) w Matches any alphanumeric character & underscore) (opposite: W). s Matches a whitespace character(opposite: S)
7. 7. Basic Rules [XYZ] Matches any single character from the character class. [XYZ]+ Matches one or more of any of the characters in the set. \$ Matches the end of the string. ^ Matches the beginning of a string. [^a-z] When inside of a character class, the ^ means NOT; in this case, it will match anything that is NOT a lowercase letter.
8. 8. Practical Examples1. In several cases, we want user to enter only alphanumericcharacters. We can achieve that functionality by using thefollowing function.function bTestOnlyAlphaNum(strToTest) { return (strToTest.match(/^[a-zA-Z0-9]+\$/) != null);}Here we are using match() function of javascript on strToTestwhich is a string.
9. 9. Practical Examplesmatch() function returns non-null value if the string matchesthe regular expression pattern, otherwise it returns null. If itreturns non-null value, our function returns true, meaning thatthe input string contained only alphanumeric characters./^[a-zA-Z0-9]+\$/is the regular expression pattern.^ specifies – from the beginning of the input string.[a-zA-Z0-9] specifies – any one character which may beany lowercase letter, uppercase letter or digit.
10. 10. Practical Examples+ specifies – one or more occurance of the previous character.\$ specifies –the end of the input string.The entire pattern collectively specifies a string that frombeginning till the end contains one or more characters whichshould be lowercase letter, uppercase letter or digit.If user enters any such string that satisfy this regular expressionpattern, match function returns non-null. Thus our functionreturns true.
11. 11. Practical Examples2. The following function matches a postal code whichcontains only digits and may be in format xxxxx orxxxxx-xxxx.function bTestPostalCode(strToTest) { if (strToTest == null || strToTest.length == 0) return false; return (strToTest.match(/^d{5}(-d{4})?\$/) != null);}Matches :1234512345-6789
12. 12. Practical Examplesd{5} specifies five digits(-d{4})? Means ”-” followed by four digits. Parenthesis areused for grouping. ”?” at the end means that this entire groupis optional.Thus the entire regular expression specifies, from the beginningof the string and till the end, there must be 5 digits followed byan optional group of - character with another 4 digits.
13. 13. Groups To match a pattern like 1234-567, we can write the regex as : /d{4}-d{3}/ In order to extract the individual portions, we can group them as follows : /(d{4})-(d{3})/ Now we can access the first four digits by 1 and last three digits by 2
14. 14. Groups Example : lets say we want to mach ”howdy123” RegEx : /[”][^”]*[”]/ But this regex does not require the opening quote to be same as closing quote. It will also match patterns like ”howdy123, which is not permitted. To prevent this we can write : /([”])[^”]*1/ Now it will match only if opening and closing quotes are same.
15. 15. search() Finds position and occurance of pattern in the string. Does not support global search. Return character position of matched pattern or -1 if no match is found. Example : ”abc 123 def 345 ghi”.search(/d{3}/) Output : 4
16. 16. match() String.match(RegExp) can perform global search and returns an array of results. For global search, the returned array contains all the matching parts of the source string. For non-global search, the returned array contains the full match along with any parenthesized sub-patterns. ”abc 123 def 345 ghi”.match(/d{3}/g) It will return an array ["123", "345"]
17. 17. replace() String.replace(RegExp,replacement) RegExp is the expression which defines the pattern to be searched for. Replacement is the text to replace the match found or is a function that generates the replacement text. If we are using global modifer with replace(), we can call a function for every match found. For every match found, the matched value will be passed to the function as first argument. If there is a group inside the matched pattern, then it will be passed as the next argument. Each matched value will be replaced with the return value from its corresponding function.
18. 18. replace()function fnReplaceWithFunction(){ var srcText=”Number one is 011-33233334, number two is 032-83993333 and finally number three is 033-37443343. Site is http://www.abc.com/index.html” var result=srcText.replace(/(d{3})-(d{8})/g, function(found,a,b) {return b;}); alert(result);}Here for every string matching the pattern, function receivesthree arguments. For example for first matched string011-33233334, found= 011-33233334, a=011 and b=33233334.
19. 19. replace()Function is returning b, thus the matched pattern 011-33233334is replaced with 33233334. Same happens for all the matchedValues. The final output is :
20. 20. Thank You