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Regular Expressions

How do they work

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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.

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Back to the subject at hand

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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.

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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,}$

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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

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Regular Expressions SyntaxCharacter 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].

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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.

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Famous Regex Engines

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Part 2

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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

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DFA

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DFA

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NFA

Equal in strength to DFASmaller in size

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Ken Thompson

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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

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Thompson NFA method

1. Check the regex and inject . For concat actiona(b|c)*d 2. Convert to reverse polish notationabc|*.d.

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Thompson NFA method cont.

char

Check single character

exp

exp

OR

exp

Kleene star

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Thompson NFA method cont.

• 3.Build the NFA

B

C A

D

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Problems for regex

• NLP

• Unicode vs. ASCII

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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.