Cyzyk, Mark

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Mark Cyzyk.  Book Review: Andrew Watt's Beginning Regular Expressions, in Technology ElectronicReviews v13 (1), February 2006. http://www.ala.org/ala/mgrps/divs/lita/ter/volume13no1.cfm#cyzykVolume 13, Number 1, February 2006Technology Electronic Reviews (TER) is a publication of the Library andInformation Technology Association.Technology Electronic Reviews (ISSN: 1533-9165) is a periodical copyright© 2006 by the American Library Association. Documents i  this issue, subjectto copyright by the American Library Association or by the authors of thedocuments, may be reproduced for noncommercial, educational, or scientificpurposes granted by Sections 107 and 108 of the Copyright Revision Act of1976, provided that the copyright statement and source for that material areclearly acknowledged and that the material is reproduced without alteration.None of these documents may be reproduced or adapted for commercialdistribution without the prior written permission of the designated copyrightholder for the specific documents.REVIEW OF: Andrew Watt. (2005). Beginning RegularExpressions. Indianapolis, IN: Wiley Publishing Inc.(ISBN: 0-7645-7489-2).by Mark CyzykMy programming career began with an attempt to compile a book-length bibliography ofdoctoral dissertations in philosophy. Such a large, textual work inevitably requiredsubstantial formatting and reformatting. In order to automate this process I learned allabout the macro language supplied with WordPerfect 5.0 for DOS. It was simple, butpowerful. As a programming novice at the time, the only routines I learned were simplesearch and replace functions. Running a very long macro on a very long piece of text on avery slow 8080 processor took a very long time, so I quickly learned that when searchingand replacing certain patterns of strings in a large textual document one must be careful.The machine, after all, is literal in a way that we humans typically are not. One of thegreat virtues of this book is that it points out the ings that can go wrong whenprogrammatically manipulating text via regular expressions and shows how thoseproblems can be circumvented or solved.But first, what is a regular expression? A "regular expression" is a pattern of characters ina text. It is "regular" insofar as it is identifiable and can therefore be used to match apattern wherever it may occur throughout a text. Now, why the terminology here isdenoted by the term "regular expression" instead of something more intuitive like "patternmatching" I don't know. But after all, we live in a technological world in which one of thekey concepts of HTTP, the foundation of the World Wide Web, is spelled "referer". Whenconfronted by the technical term "regular expression" I simply do an immediate internaltranslation to "pattern matching", and all is well.This book consists of twenty-six chapters, the first ten illustrate the various aspects ofregular expression technology, the remaining sixteen provide useful illustrations of howregular expressions are implemented in the top programming languages and applications.The core of the book is to be found in Chapters 3 through 9.Chapter 3: Simple Regular ExpressionsThis chapter introduces and illustrates how to write a regular expression for use inmatching patterns in text where the pattern to be matched consists of single characters,multiple characters in a string, optional characters, as well as using special"metacharacters", characters that have a special meaning within a regular expression tospecify such things as match a class of characters; match any character; match within acertain range of characters; match a pattern zero or more times; match a pattern one ormore times; and match a pattern a specified minimum and maximum number of timeswithin a string of text. This is an introductory chapter to these topics which sets the stagefor the chapters that follow.Chapter 4: Metacharacters and ModifiersA regular expression can be very simple, something for example like this: cat. This simpleregular expression would match any sequence of a c followed by an a followed by a t in astring of text. So it would match that string as it appears in the words cat, cats, catalog,catastrophe, concatenation, etc. Metacharacters provide for more sophisticated patternmatching. Suppose for example you wanted to find all instances of cat or cut in a text.The period (.) metacharacter would allow you to do so: c.t. The period metacharactermatches any character. This chapter illustrates the use of the following metacharacters:. Matches any character\wMatches characters in the English alphabet, numeric characters, and theunderscore\W Matches any character not matched by \w\d Matches a numeric digit\D Matches any character not matched by \d\s Matches a single space, a tab, or a newline charater\S Matches any character not matched by \s\t Matches a single tab character\nMatches a single newline character escaped characters. Characters likethe period and the backslash themselves must be "escap d" with abackslash: \. and \Chapter 5: Character ClassesCharacter classes are convenient ways to match a range or entire classes of characters in atext. For instance:[a-z] Matches any single lowercase alphabetic characte[a-z]+ Matches any number of lowercase alphabetic characters[A-Z] Matches any single uppercase alphabetic characte[0-9] Matches any numeric digit[aeiou] Matches any vowelParentheses can be used to group items:(33|44) Matches either a 33 or a 44 in a textUse of quantifiers specify how many characters to match:[aeiou]{2} Matches any two consecutive vowelsNegated character classes can be specified with the metacharacter :[^a-z] Matches any single character that is not a lowercase alphabeticChapter 6: String, Line, and Word BoundariesMetacharacter "anchors" are used to denote start and end of lines, and beginning and endsof words:^ Matches the start of a line of text$ Matches the end of a line of text^$ Matches a blank line\b Matches either the beginning or the ending of a word\<< /> Matches the beginning of a word\> Matches the ending of a wordChapter 7: Parentheses in Regular ExpressionsJust as one uses parentheses to group elements in a Boolean search query, so too are theyused in regular expressions to group parts of the expression.(Doctor|Dr|Dr\.) Matches "Doctor" "Dr" or "Dr."Watt briefly discusses, in this chapter, the notion of "capturing" parentheses, somethingthat strikes me as being extraordinarily powerful. Essentially, programming languagesand applications that allow for capturing parenthesis will match what is inside theparentheses and assign what is found there to a numbered variable name. The exampleWatt gives is:(United) (States)as used against the text:The United StatesIn this case, the first parenthetical expression will match the string "United" in the textand will assign the value found there to a variable named "$1". The second parentheticalexpression will match against the string "States" in the text and will assign the valuefound there to a variable named "$2". These two variables are now available for use bythe programmer.Chapter 8: Lookahead and LookbehindAnother powerful feature of regular expressions is the ability to match a pattern based ona previously matched pattern or based on a pattern being followed by another pattern.This is called lookahead and lookbehind. There are sev ral different flavors of these twofeatures, but just as an example, here is what Watt uses to illustrate a "positivelookahead":Star(?=Training)Basically this regular expression says "Find the string 'Star', but only if it is immediatelyfollowed by the string 'Training'".Lookahead and lookbehind, with their variants, surely are powerful programmingconstructs.Chapter 9: Sensitivity and Specificity in Regular ExpressionsThe moral of this instructive chapter is: Sensitivity and specificity in a regular expressionact in mutually opposing ways such that an increase in the sensitivity of an expressionmost often results in the decrease of specificity, and vice versa. So increasing eitherusually results in a tradeoff: The more sensitive your search, the more "false hits" arelikely to result; the more specific your search, the more likely it will result in relevantitems being excluded. Watt provides solid examples of this principle in action.Chapters 11-26 discuss and illustrate the implementatio  of regular expressions in suchapplications, application programming platforms, and programming languages as:Microsoft Word; StarOffice/OpenOffice; findstr; PowerGREP; Microsoft Excel; SQLServer 2000; MySQL; Microsoft Access; JScript and Javascript; VBScript; VisualBasic .NET; C#; W3C XML Schema; Java; and Perl.A chapter on regular expressions and Python would have been nice, but I suppose the linehad to be drawn somewhere and an already very large book (742 pages) brought to anend. The other thing I would have liked to see is the provision of an Appendix brieflysummarizing the syntax of regular expressions, a visual summary of the first severalchapters -- sort of a Quick Lookup area of the book that could be easily accessible to theworking programmer.Overall, the book is informative, comprehensive, well- ritten, and is therefore easilyreadable. The examples are well thought out and clear. I think this book is ideal forsomeone starting out with regular expression programming and it also would have greatvalue to the programmer who, for instance, needs to switch programming languages,from, say, Java or Perl to PHP.In the end it's all about solving programming problems. Suppose, for instance, you aregiven a file of text consisting of what appears to be IP addresses, each on its own line, butyou need to match just the strings that actually are v lid IPs and weed out the rest. How todo this?It's simple - just match on the following regular expression!:^((25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]|[0-9])\.){3}(24[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]|[0-9])$Mark Cyzyk, formerly Head of Information Technology in the Albert S. Cook Library atTowson University, is currently the Web Architect at Johns Hopkins University inBaltimore, Maryland.Copyright © 2006 by Mark Cyzyk. This document may be reproduced in whole or in partfor noncommercial, educational, or scientific purposes, provided that the precedingcopyright statement and source are clearly acknowledged. All other rights are reserved.For permission to reproduce or adapt this document or any part of it for commercialdistribution, address requests to the author at mcyzyk@jhu.edu.About TEREditor is Sharon Rankin, McGill University (sharon.rankin@mcgill.ca).Editorial Board:• Frank Cervone, Northwestern University (f-cervone@northwestern.edu) • Tierney Morse McGill, Colorado State University (mcgill@manta.colostate.edu) • Florence Tang, Mercer University, Atlanta (ng_fy@mercer.edu) • Laura Wrubel, University of Maryland (lwrubel@umd.edu) • Michael Yukin, University of Nevada, Las Vegas(michael.yunkin@ccmail.nevada.edu) Technology Electronic Reviews (TER) is an irregular electronic serial publication of the Library andInformation Technology Association, a division of the American Library Association, 50 E. Huron St.,Chicago, IL 60611. The primary function of TER is to provide reviews of and pointers to a variety ofprint and electronic resources about information technology. Resources include books, articles, serials,discussion lists, training materials, bibliographies, and other items of interest to librarians andinformation technology professionals. The topics covered may include, but are not limited to,networking technologies and standards; hardware and software; operating systems; databases; specificprogramming languages; management tools and utilities; echnical project management; training andpersonnel issues; library perspectives; and research and development.Opinions expressed in this publication are those of the writers and do not necessarily represent theviewpoints of LITA, ALA, or organizations involved in the storage and/or distribution of thepublication. TER is distributed electronically via Internet. There is no subscription fee. Currently it is available viaWorld Wide Web (http://www.lita.org/ter/) and new-issue announcements are posted on the LITA-Lelectronic discussion list. To subscribe, send an email message to listproc@ala1.ala.org that says:subscribe LITA-L First-Name Last-Name. Other distribut on arrangements may be made in the future.

Book Review: Andrew Watt's Beginning Regular Expressions

Johns Hopkins University

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Book Review: Andrew Watt's Beginning Regular Expressions

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