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Regular Expression Crash Course


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these are slides used to explain Regular Expression on my Channel link in Urdu / Hindi language and if you are interested kindly watch that video along with these slides.

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Regular Expression Crash Course

  1. 1. REGULAR EXPRESSION IN ACTION Brief overview of Regular Expression building blocks and tools with a practical example
  3. 3. THE REGEX COACH IS A GRAPHICAL APPLICATION FOR WINDOWS WHICH CAN BE USED TO EXPERIMENT WITH REGULAR EXPRESSIONS INTERACTIVELY. ◦ Sublime Text is a text editor that has support of find and replace using Regular Expressions. Web based Regular Expressions tester. ◦
  4. 4. THE MOST BASIC REGULAR EXPRESSION CONSISTS OF A LITERAL which behaves just like string matching. For e.g. ◦ cat will match cat in About cats and dogs. Special characters known as meta characters needs to be escaped with a in regular expressions if they are used as part of a literal: ◦ dogs.will match dogs. in About cats and dogs. Meta characters are: ◦ [ ^ $ . | ? * + ( ) {
  5. 5. WITH A "CHARACTER CLASS", ALSO CALLED "CHARACTER SET", YOU CAN TELL THE REGEX ENGINE TO MATCH ONLY ONE OUT OF SEVERAL CHARACTERS. FOR E.G. ◦ gr[ae]ywill match grey and gray both. Ranges can be specified using dash. For e.g. ◦ [0-9]will match any digit from 0 to 9. ◦ [0-9a-fA-F]will match any single hexadecimal digit. Caret after the opening square bracket will negate the character class. • The result is that the character class will match any character that is not • in the character class. For e.g. ◦ [^0-9] will match any thing except number. ◦ q[^u] will not match Iraq but it will match Iraq is a country
  6. 6. Meta characters works fine without escaping in Character classes. For e.g. ◦ [+*]is a valid expression and match either * or +. There are some pre-defined character classes known as short hand character classes: ◦ w stands for[A-Za-z0-9_] ◦ s stands for[ trn] ◦ d stands for[0-9] If a character class is repeated by using the ?, * or + operators, the entire character class will be repeated, and not just the character that it matched. For e.g. ◦ [0-9]+ can match 837 as well as 222 ◦ ([0-9])1+ will match 222 but not 837.
  7. 7. The famous dot “.” operator matches anything. For e.g. ◦ a.b will match abb, aab, a+b etc. ^ and $ are used to match start and end of regular expressions. For e.g. ◦ ^My.*.$ will match anything starting with My and ending with a dot. Pipe operator is used to match a string against either its left or the right part. For e.g. ◦ (cat|dog) can match both cat or dog. Question: ◦ If the expression is Get|GetValue|Set|SetValue and string is SetValue. What will this match and why? ◦ What if the expression becomes Get(Value)?|Set(Value)? * or {0,} and+ or {1,} are used to control repititions.
  8. 8. Round brackets besides grouping part of a regular expression together, also create a "backreference". A backreference stores the matching part of the string matched by the part of the regular expression inside the parentheses. For e.g. ◦ ([0-9])1+ will match 222 but not 837. If backreference are not required, you can optimize this regular expression Set(?:Value)? Backreferences can be used in expressions itself or in replacement text. For e.g. ◦ <([A-Za-z][A-Za-z0-9]*)>.*</1>will match matching opening and closing tags.
  9. 9. /i makes the regex match case insensitive. ◦ [A-Z] will match A and a with this modifier. /s enables "single-line mode". In this mode, the dot matches newlines as well. ◦ .* will match sherazrnattari with this modifier. /m enables "multi-line mode". In this mode, the caret and dollar match before and after newlines in the subject string. ◦ .* will match only sherazin sherazrnattari with this modifier. /x enables "free-spacing mode". In this mode, whitespace between regex tokens is ignored, and an unescaped # starts a comment. ◦ #sherazrnrn.* will match only sheraz in with this modifier.
  10. 10. A conditional is a special construct that will first evaluate a lookaround, and then execute one sub-regex if the lookaround succeeds, and another sub-regex if the lookaround fails. Example of Positive lookahead is: ◦ q(?=uv*)will match q in quvvvv and qu. Example of Negative lookahead is: ◦ q(?!uv*)will match q not followed by u and uv. Example of Positive lookbehind is: ◦ (?<=b)awill match a prefixed by b like ba. Example of Negative lookbehind is: ◦ (?<!b)awill match a not prefixed by b like ca and da etc.
  11. 11. abc… Letters 123… Digits d Any Digit D Any Non-digit character . Any Character . Period [abc] Only a, b, or c [^abc]Not a, b, nor c [a-z] Characters a to z [0-9] Numbers 0 to 9 w Any Alphanumeric character W Any Non-alphanumeric character {m} m Repetitions {m,n} m to n Repetitions * Zero or more repetitions + One or more repetitions ? Optional character s Any Whitespace S Any Non-whitespace character ^…$ Starts and ends (…) Capture Group (a(bc)) Capture Sub-group (.*) Capture all (abc|def) Matches abc or def
  12. 12. Most of the content is taken from THANK YOU!