All About Regular Expressions

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ALL ABOUT REGULAR EXPRESSIONS JADE ALLEN @AVOCADOSUPERFAN

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OVERVIEW • ORIGIN, THEORY AND BASIC ALGEBRA • DETERMINISTIC AND NONDETERMINISTIC FINITE AUTOMATA • WAIT. WHAT? WHY ARE WE TALKING ABOUT EDITORS? • FERAL REGULAR EXPRESSIONS

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MCCULLOCH-PITTS NEURON MODEL

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STEPHEN KLEENE

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ALGEBRA OF REGULAR EVENTS

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DETERMINISTIC FINITE AUTOMATA •EACH STATE TRANSITION IS UNIQUELY DETERMINED BY THE SOURCE STATE AND INPUT SYMBOL •READING AN INPUT SYMBOL IS REQUIRED FOR EACH STATE TRANSITION •ADVANTAGE: EASY TO IMPLEMENT AS A PROGRAM •DRAWBACK: STATE EXPLOSION

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REGULAR EXPRESSION SEARCH ALGORITHM

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Σ = { A, B, C, D } A(B|C)*D

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WILL IT MATCH? •INPUT: AD – MATCH? •INPUT: ABBBBBBBB – MATCH? •INPUT: ADCD – MATCH? 🤔

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WILL IT MATCH? •INPUT: AD – MATCH? ✅ •INPUT: ABBBBBBBB – MATCH? •INPUT: ADCD – MATCH? 🤔

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WILL IT MATCH? •INPUT: AD – MATCH? •INPUT: ABBBBBBBB – MATCH? •INPUT: ADCD – MATCH? 🤔

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WILL IT MATCH? •INPUT: AD – MATCH? •INPUT: ABBBBBBBB – MATCH? ✅ •INPUT: ADCD – MATCH? 🤔

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WILL IT MATCH? •INPUT: AD – MATCH? •INPUT: ABBBBBBBB – MATCH? •INPUT: ADCD – MATCH? 🤔

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WILL IT MATCH? •INPUT: AD – MATCH? •INPUT: ABBBBBBBB – MATCH? •INPUT: ADCD – MATCH? ❌ 🤔

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HMM…

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INPUT / OUTPUT FORMATS BEFORE TIME SHARING

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AN ONLINE EDITOR

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FROM WHERE DOES THE SYNTAX COME?

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EGREP (1978) • TURING AWARD WINNER 2020 • AWK (LEX, YACC)

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I HAVE NO ^ AND I MUST MATCH BACK IN THE DAY, NO ONE HAD A “STANDARD KEYBOARD” (*NOR A STANDARD DISPLAY TERMINAL, BUT THAT IS A FUTURE TALK) YOU GET WHAT YOU GOT, AND YOU WERE GRATEFUL EURO-FLAVORED KEYBOARDS MAY OR MAY NOT HAVE “STANDARD” US ASCII CHARACTERS

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INTO THE WILD… HENRY SPENCER WROTE AND PUBLISHED AN EGREP COMPATIBLE “NEARLY PUBLIC DOMAIN” REGULAR EXPRESSION LIBRARY IN 1986

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PERL (1987)

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MAXIMALIST REGEX “TOO MUCH IS NEVER ENOUGH!”

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PCRE

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REGEX CONSIDERED HARMFUL

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RE2

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SUMMARY Regular expressions originated from mathematical concepts in matching sequences from sets of symbols from alphabets Implementations came from editors and compilers Unix provided facilities to use regex from tools Awk provided a way to use them to trigger actions in a dynamic script Perl added a bunch of features people liked …until they didn’t

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THANK YOU!

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BIBLIOGRAPHY • [1] MCCULLOCH, W.S. AND PITTS, W. A LOGICAL CALCULUS OF THE IDEAS IMMANENT IN NERVOUS ACTIVITY, BULLETIN OF MATHEMATICAL BIOLOGY, 1943 • [2] KLEENE, S.C., REPRESENTATION OF EVENTS IN NERVE NETS AND FINITE AUTOMATA, RAND PROJECT MEMO, RM-704, 1951. • [3] RABIN, M. O. AND SCOTT, D. FINITE AUTOMATA AND THEIR DECISION PROBLEMS, IBM JOURNAL OF RESEARCH AND DEVELOPMENT, 1959. • [4] THOMPSON, K., REGULAR EXPRESSION SEARCH ALGORITHM, CACM, JUNE 1968.

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BIBLIOGRAPHY • [5] DEUTSCH, P., AND LAMPSON, B., AN ONLINE EDITOR, CACM, DECEMBER 1967. • [6] RITCHIE, D., AND THOMPSON, K., QED TEXT EDITOR, BELL LABS TECHNICAL MEMORANDUM, 1970. • [7] AHO, A., AND CORASICK, M. EFFICIENT STRING MATCHING: AN AID TO BIBLIOGRAPHIC SEARCH, CACM, JUNE 1975 • [8] SPENCER, H. USENIX POST, JANUARY 1986, REGEXP(3) (GOOGLE.COM).

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BIBLIOGRAPHY • [9] FRIEDL, J, MASTERING REGULAR EXPRESSIONS, O’REILLY, 1997. • [10], HAZEL, P., FROM PUNCHED CARDS TO FLAT SCREENS, CIHK.PDF - GOOGLE DRIVE ,2017. • [11] COX, R. REGULAR EXPRESSION MATCHING CAN BE SIMPLE AND FAST (SWTCH.COM), 2007. • [12], COX, R. REGULAR EXPRESSION MATCHING IN THE WILD (SWTCH.COM), 2010.