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Regular expressions

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Brij Kishore

The document discusses regular expressions (regex), which are symbols used to match patterns in text. It provides examples of regex for identifiers, passwords, emails, IPv4 addresses, and IPv6 addresses. It also covers the basics of regex syntax, including characters, quantifiers, anchors, character classes, groups and backreferences. The document notes that regex are used for text matching, searching and replacing. It lists several programming languages and tools that support regex and provides areas where regex are commonly applied, such as validation and pattern matching.

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REGEXThe Black Magic Of Programming Submitted By Brij Raj Kishore
Motivation For REGEX System Administrators Developers QA Engineers Support Engineers User ^[A-Za-z][A-Za-z0-9_]*$ for identifiers Pseduo-Code state = FIRSTCHAR for char in all_chars_in(string): if state == FIRSTCHAR: if char is not in the set "A-Z" or "a-z": error "Invalid first character" state = SUBSEQUENTCHARS next char if state == SUBSEQUENTCHARS: if char is not in the set "A-Z" or "a-z" or "0- 9" or "_": error "Invalid subsequent character" state = SUBSEQUENTCHARS next char
What is regex Called by different names :- Regular Expression, Regexp, Regex, ASCII puke. /^Reg(exp?|ular expression)$/i Symbols representing a text pattern. Formal language interpreted by a regular expression processor. Used for matching, searching, and replacing text. Not a programming language. Formal Definition Type 3 grammar in Chomsky Classification of Language Recognized by FSA.
Type 3 Grammer Languages defined byType-3 grammars are accepted by finite state automata Rules are of the form: A 竜 A 留 A 留B where A, B N and 留 裡
Examples List of all pdf files abc.pdf, 123.pdf, _12abc.pdf /^w+.pdf$ A 6+ letter password with at least: one number, one letter and one symbol -: abc@1236 /^(?=.*d)(?=.*[a-z])(?=.*[W_]).{6,}$/i Email format such as -: Aaavv_ahchj@bbb.cccc /^[a-zA-z0-9_]+@[a-zA-Z0-9]+.[a-zA-z]{2,}$/ Ipv4 validation Format is 0.0.0.0 to 255.255.255.255 ^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$
Regular expressions engines C/C++ Java JavaScript/ActionScript(ECMAScript) .NET Perl PHP(PCRE) Python Ruby Unix(POSIX BRE, POSIX ERE) Apache (v1: POSIX ERE, v2: ERE) MySQL etc.
Syntax Of Regular expression g/Regular expression/p It is case sensitive by default unless specified by the i flag. Regular Symbols contains :- Character Symbols Non-characters Metacharacters Some Rules
Symbols and their meanings abc.. Letters 123 Digits d Any Digit D Any Non-Digit characters . Any Character Period Period [abc] only a,b or c [^abc] Not a,b nor c [0-9] Number 0-9 w Any Alphanumeric character b Boundary Between AWord And NonWord Character B Boundary BetweenWord &Word Or Non-Word & Non-Word.
W Any Non-Alphanumeric character {m} m repetitions {m,n} m to n Repetitions * Zero or more repetitions + One or more repetitions ? OptionalCharacter s Any Whitspaces S Any Non-whitespace character ^..$ Starts and end (.) Capture Group (a(bc)) Capture Sub-group (.*) Capture all (abc|def) Matches abc or def
Some Key Points ., .*, .+, .?, .{1,2} - Arbitrary characters and repetitions ^, $ - Start and end of subject (or line in multiline mode) foo|bar - LogicalOr (foo)(bar) - Subpattern grouping /(foo|bar)baz1/ - Backreferences [a-zA-Z] - Character classes. Negative assertions, Positive assertions are possible -: a(?!x) , a(?=x) foo not followed by another foo -: /foo(?!foo)/ foo followed by another foo -: /foo(?=foo)/ If condition - /(?(condition)yes-pattern|no-pattern)/
Application Areas Validation Framework Pattern Matching Translation Program DigitalCircuits Protocols
Ipv6 validation (?:^|(?<=s))(([0-9a-fA-F]{1,4}:){7,7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA- F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA- F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA- F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:((:[0-9a-fA-F]{1,4}){1,6})|:((:[0-9a-fA- F]{1,4}){1,7}|:)|fe80:(:[0-9a-fA-F]{0,4}){0,4}%[0-9a-zA-Z]{1,}|::(ffff(:0{1,4}){0,1}:){0,1}((25[0- 5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]).){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])|([0-9a-fA- F]{1,4}:){1,4}:((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]).){3,3}(25[0-5]|(2[0-4]|1{0,1}[0- 9]){0,1}[0-9]))(?=s|$) Sometimes you need to know where to stop Allow some false positive rather than some false negative Short hand for ipv6 regex ^((?:[0-9A-Fa-f]{1,4}(?::[0-9A-Fa-f]{1,4})*)?)::((?:[0-9A-Fa-f]{1,4}(?::[0-9A-Fa-f]{1,4})*)?)$
For Performance Avoid greedy quantifiers :- /(x+x)+y/ xxxxxxxxxxxxxxxxxxxx Dont forget anchors (^ and $) Be as specific as possible Prefer non-capturing groups ( ?: ) Minimize backtracking
References Websites https://regexone.com/ https://github.com http://stackoverflow.com/questions/5284147/validating-ipv4-addresses-with-regexp https://www.youtube.com/watch?v=EkluES9Rvak https://www.youtube.com/watch?v=Ju8EybDDjBk Regular Expression Lynda Books :- Oreilly Mastering Regular Expressions 3rd Edition Aug 2006 - Jeffrey E.F. Friedl Regular Expression Pocket Reference, 2nd Edition - O'Reilly Media
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Regular expressions

  • 1. REGEXThe Black Magic Of Programming Submitted By Brij Raj Kishore
  • 2. Motivation For REGEX System Administrators Developers QA Engineers Support Engineers User ^[A-Za-z][A-Za-z0-9_]*$ for identifiers Pseduo-Code state = FIRSTCHAR for char in all_chars_in(string): if state == FIRSTCHAR: if char is not in the set "A-Z" or "a-z": error "Invalid first character" state = SUBSEQUENTCHARS next char if state == SUBSEQUENTCHARS: if char is not in the set "A-Z" or "a-z" or "0- 9" or "_": error "Invalid subsequent character" state = SUBSEQUENTCHARS next char
  • 3. What is regex Called by different names :- Regular Expression, Regexp, Regex, ASCII puke. /^Reg(exp?|ular expression)$/i Symbols representing a text pattern. Formal language interpreted by a regular expression processor. Used for matching, searching, and replacing text. Not a programming language. Formal Definition Type 3 grammar in Chomsky Classification of Language Recognized by FSA.
  • 4. Type 3 Grammer Languages defined byType-3 grammars are accepted by finite state automata Rules are of the form: A 竜 A 留 A 留B where A, B N and 留 裡
  • 5. Examples List of all pdf files abc.pdf, 123.pdf, _12abc.pdf /^w+.pdf$ A 6+ letter password with at least: one number, one letter and one symbol -: abc@1236 /^(?=.*d)(?=.*[a-z])(?=.*[W_]).{6,}$/i Email format such as -: Aaavv_ahchj@bbb.cccc /^[a-zA-z0-9_]+@[a-zA-Z0-9]+.[a-zA-z]{2,}$/ Ipv4 validation Format is 0.0.0.0 to 255.255.255.255 ^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$
  • 6. Regular expressions engines C/C++ Java JavaScript/ActionScript(ECMAScript) .NET Perl PHP(PCRE) Python Ruby Unix(POSIX BRE, POSIX ERE) Apache (v1: POSIX ERE, v2: ERE) MySQL etc.
  • 7. Syntax Of Regular expression g/Regular expression/p It is case sensitive by default unless specified by the i flag. Regular Symbols contains :- Character Symbols Non-characters Metacharacters Some Rules
  • 8. Symbols and their meanings abc.. Letters 123 Digits d Any Digit D Any Non-Digit characters . Any Character Period Period [abc] only a,b or c [^abc] Not a,b nor c [0-9] Number 0-9 w Any Alphanumeric character b Boundary Between AWord And NonWord Character B Boundary BetweenWord &Word Or Non-Word & Non-Word.
  • 9. W Any Non-Alphanumeric character {m} m repetitions {m,n} m to n Repetitions * Zero or more repetitions + One or more repetitions ? OptionalCharacter s Any Whitspaces S Any Non-whitespace character ^..$ Starts and end (.) Capture Group (a(bc)) Capture Sub-group (.*) Capture all (abc|def) Matches abc or def
  • 10. Some Key Points ., .*, .+, .?, .{1,2} - Arbitrary characters and repetitions ^, $ - Start and end of subject (or line in multiline mode) foo|bar - LogicalOr (foo)(bar) - Subpattern grouping /(foo|bar)baz1/ - Backreferences [a-zA-Z] - Character classes. Negative assertions, Positive assertions are possible -: a(?!x) , a(?=x) foo not followed by another foo -: /foo(?!foo)/ foo followed by another foo -: /foo(?=foo)/ If condition - /(?(condition)yes-pattern|no-pattern)/
  • 11. Application Areas Validation Framework Pattern Matching Translation Program DigitalCircuits Protocols
  • 12. Ipv6 validation (?:^|(?<=s))(([0-9a-fA-F]{1,4}:){7,7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA- F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA- F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA- F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:((:[0-9a-fA-F]{1,4}){1,6})|:((:[0-9a-fA- F]{1,4}){1,7}|:)|fe80:(:[0-9a-fA-F]{0,4}){0,4}%[0-9a-zA-Z]{1,}|::(ffff(:0{1,4}){0,1}:){0,1}((25[0- 5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]).){3,3}(25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9])|([0-9a-fA- F]{1,4}:){1,4}:((25[0-5]|(2[0-4]|1{0,1}[0-9]){0,1}[0-9]).){3,3}(25[0-5]|(2[0-4]|1{0,1}[0- 9]){0,1}[0-9]))(?=s|$) Sometimes you need to know where to stop Allow some false positive rather than some false negative Short hand for ipv6 regex ^((?:[0-9A-Fa-f]{1,4}(?::[0-9A-Fa-f]{1,4})*)?)::((?:[0-9A-Fa-f]{1,4}(?::[0-9A-Fa-f]{1,4})*)?)$
  • 13. For Performance Avoid greedy quantifiers :- /(x+x)+y/ xxxxxxxxxxxxxxxxxxxx Dont forget anchors (^ and $) Be as specific as possible Prefer non-capturing groups ( ?: ) Minimize backtracking
  • 14. References Websites https://regexone.com/ https://github.com http://stackoverflow.com/questions/5284147/validating-ipv4-addresses-with-regexp https://www.youtube.com/watch?v=EkluES9Rvak https://www.youtube.com/watch?v=Ju8EybDDjBk Regular Expression Lynda Books :- Oreilly Mastering Regular Expressions 3rd Edition Aug 2006 - Jeffrey E.F. Friedl Regular Expression Pocket Reference, 2nd Edition - O'Reilly Media