Regular expressions
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Regular expressions provide a concise way to match patterns in text. They work by converting the regex into a state machine that can efficiently search a string to find matches. Important regex syntax includes quantifiers like *, +, ?, character classes like [a-z], and anchors like ^ and $. Regular expression engines turn the regex pattern into a program that can search strings. Thompson's NFA construction algorithm is commonly used to build the state machine from a regex for efficient matching.
![Why do we need regular expressions
(in programming)
Many reasons but most of them are in their base
finding strings in text .
Preferably without reading it
^(?("")(""[^""]+?""@)|(([0-9a-z]((.(?!.))|[-
!#$%&'*+/=?^`{}|~w])*)(?<=[0-9a-
z])@))(?([)([(d{1,3}.){3}d{1,3}])|(([0-9a-z][-
w]*[0-9a-z]*.)+[a-z0-9]{2,17}))$
^(?=.*[^a-zA-Z])(?=.*[a-z])(?=.*[A-Z])S{8,}$](https://image.slidesharecdn.com/regularexpressions-130111001242-phpapp02/85/Regular-expressions-5-320.jpg)
![Regular Expressions Syntax
meta characters
Grouping
. – match any other character
[ ] – grouping, match single character that is inside the group
[^ ] – grouping, match single character that is not inside the group
( ) – sub expression, in Perl can be recalled later from special variables
Quantifier
{m,n} –specifies that the character/sub expression before need to be matched
at least m times and no more than n times
* - derived from Kleene star in formal logic, matches 0 or more amount of the
character before it.
? –matches zero or one of the preceding elements
+ - derived from Kleene cross in formal logic, matches 1 or more of the
character before it.
Location
^ - Marking start of line
$ - Marking end of line](https://image.slidesharecdn.com/regularexpressions-130111001242-phpapp02/85/Regular-expressions-6-320.jpg)
![Regular Expressions Syntax
Character groups
[:alpha:] - Any alphabetical character - [A-Za-z]
[:alnum:] - Any alphanumeric character - [A-Za-z0-9]
[:ascii:] - Any character in the ASCII character set.[:blank:] - A GNU
extension, equal to a space or a horizontal tab ("t")
[:cntrl:] - Any control character
[:digit:] - Any decimal digit - [0-9], equivalent to "d“
[:graph:] - Any printable character, excluding a space
[:lower:] - Any lowercase character - [a-z]
[:print:] - Any printable character, including a space
[:punct:] - Any graphical character excluding "word" characters
[:space:] - Any whitespace character. "s" plus the vertical tab ("cK")
[:upper:] - Any uppercase character - [A-Z]
[:word:] - A Perl extension - [A-Za-z0-9_], equivalent to "w“
[:xdigit:] - Any hexadecimal digit - [0-9a-fA-F].](https://image.slidesharecdn.com/regularexpressions-130111001242-phpapp02/85/Regular-expressions-7-320.jpg)
![Some examples of Regex
• ([^s]+(.(?i)(jpg|png|gif|bmp))$)
– Match file with specific extentions
• ^(https?://)?([da-z.-]+).([a-z.]{2,6})([/w
.-]*)*/?$
– Match URL
• /^#?([a-f0-9]{6}|[a-f0-9]{3})$/
– Match a hex value
• [ -~]
– An interesting one.](https://image.slidesharecdn.com/regularexpressions-130111001242-phpapp02/85/Regular-expressions-21-320.jpg)
Regular expressions
- 1. Regular Expressions How do they work
- 2. Several important Facts 1. Everything in computing was discovered in one form or another in the 70-80’s and was probably thought about during the 60’s. 2. The easiest way to become a great computer engineer in the 80’s was to work for Bell Labs and have a beard.
- 3. Back to the subject at hand
- 4. What are regular expressions? From Wikipedia: In computing, a regular expression provides a concise and flexible means to "match" (specify and recognize) strings of text, such as particular characters, words, or patterns of characters. Common abbreviations for "regular expression" include regex and regexp.
- 5. Why do we need regular expressions (in programming) Many reasons but most of them are in their base finding strings in text . Preferably without reading it ^(?("")(""[^""]+?""@)|(([0-9a-z]((.(?!.))|[- !#$%&'*+/=?^`{}|~w])*)(?<=[0-9a- z])@))(?([)([(d{1,3}.){3}d{1,3}])|(([0-9a-z][- w]*[0-9a-z]*.)+[a-z0-9]{2,17}))$ ^(?=.*[^a-zA-Z])(?=.*[a-z])(?=.*[A-Z])S{8,}$
- 6. Regular Expressions Syntax meta characters Grouping . – match any other character [ ] – grouping, match single character that is inside the group [^ ] – grouping, match single character that is not inside the group ( ) – sub expression, in Perl can be recalled later from special variables Quantifier {m,n} –specifies that the character/sub expression before need to be matched at least m times and no more than n times * - derived from Kleene star in formal logic, matches 0 or more amount of the character before it. ? –matches zero or one of the preceding elements + - derived from Kleene cross in formal logic, matches 1 or more of the character before it. Location ^ - Marking start of line $ - Marking end of line
- 7. Regular Expressions Syntax Character groups [:alpha:] - Any alphabetical character - [A-Za-z] [:alnum:] - Any alphanumeric character - [A-Za-z0-9] [:ascii:] - Any character in the ASCII character set.[:blank:] - A GNU extension, equal to a space or a horizontal tab ("t") [:cntrl:] - Any control character [:digit:] - Any decimal digit - [0-9], equivalent to "d“ [:graph:] - Any printable character, excluding a space [:lower:] - Any lowercase character - [a-z] [:print:] - Any printable character, including a space [:punct:] - Any graphical character excluding "word" characters [:space:] - Any whitespace character. "s" plus the vertical tab ("cK") [:upper:] - Any uppercase character - [A-Z] [:word:] - A Perl extension - [A-Za-z0-9_], equivalent to "w“ [:xdigit:] - Any hexadecimal digit - [0-9a-fA-F].
- 8. What is a regular expression engine A regular expression engine is a program that takes a set of constraints specified in a mini- language, and then applies those constraints to a target string, and determines whether or not the string satisfies the constraints. In less grandiose terms, the first part of the job is to turn a pattern into something the computer can efficiently use to find the matching point in the string, and the second part is performing the search itself.
- 10. Part 2
- 11. How the Perl Regex engine works • Unlike the army only two steps – Compilation • Parsing (Size, Construction) • Peep-hole optimization and analysis – Execution • Start position and no-match optimizations • Program execution
- 12. DFA
- 13. DFA
- 14. NFA Equal in strength to DFA Smaller in size
- 15. Ken Thompson
- 16. Thompson NFA method • In 1968 Thompson wrote an article on how to convert a regular expression to still unnamed automata (NFA) • The article included code to explain the point
- 17. Thompson NFA method 1. Check the regex and inject . For concat action a(b|c)*d 2. Convert to reverse polish notation abc|*.d.
- 18. Thompson NFA method cont. Check single character OR char exp exp Kleene star exp
- 19. Thompson NFA method cont. • 3.Build the NFA B A C D
- 20. Problems for regex • NLP • Unicode vs. ASCII
- 21. Some examples of Regex • ([^s]+(.(?i)(jpg|png|gif|bmp))$) – Match file with specific extentions • ^(https?://)?([da-z.-]+).([a-z.]{2,6})([/w .-]*)*/?$ – Match URL • /^#?([a-f0-9]{6}|[a-f0-9]{3})$/ – Match a hex value • [ -~] – An interesting one.
Editor's Notes
- #3: Where they worked on MULTICS which later became unix
- #4: From XKCD
- #6: The first is for email from MSDN the second is for a password of 8 characters with atleast one lowercase upper case and other symbol
- #7: This is from the POSIX specification
- #8: Again those are the POSIX groups which are not implemented everywhere the examples are for Perl regex
- #10: Camel for Perl, Japaneas for Onigurama (Devil Chariot), Henry Spencers picture, PCRE logo, google for RE2 and a gnu for the GNU REGEX
- #12: In compiler theory, peephole optimization is a kind of optimization performed over a very small set of instructions in a segment of generated code. The set is called a "peephole" or a "window". It works by recognising sets of instructions that can be replaced by shorter or faster set of instructions.The execution is including other algorithms like Bayer-Moore and other ways to shorten execution timeFunctions – compilationReg() for parsingReg_branch() Reg_piece()Reg_atom()Reg_tail()Study_chunk() for optimizationFunctions – ExecutionRe_intuit_start() – for starting locationsReg_try()Regmatch()
- #13: Not the department of foreign affairs
- #14: Accepts the language of all numbers ends with 10 or 01
- #15: NFA’s transition table for the regex (l|e)*n?(i|e)el*
- #16: Thompson on the left and ritchy on the rightThompson invented B helped inventing CAlso created worked on QED and ed, and invented regular expressionsHelped develop utf8 and the go language
- #17: The original article included algol code for the IBM mainframe and was implanted in QED editor
- #18: Reverse polish notation is used to remove ambiguity and make it easier to work with stack
- #21: Natural language processing is becoming common due to computer speed and massive amounts of data, using statistical tools.UNICODE VS ASCII is a problem for developer due to characters that might be cought as the same while being different
- #22: Notice it doesn’t use the xdigit which is not supported in all engines