book

Mastering Regular Expressions, 3rd Edition

by Jeffrey E.F. Friedl

August 2006

Intermediate to advanced

542 pages

15h 33m

English

O'Reilly Media, Inc.

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Solving Real ProblemsRegular Expressions as a LanguageThe Filename AnalogyThe Language AnalogyThe Regular-Expression Frame of MindIf You Have Some Regular-Expression ExperienceSearching Text Files: EgrepEgrep MetacharactersStart and End of the LineCharacter ClassesMatching Any Character with DotAlternationIgnoring Differences in CapitalizationWord BoundariesIn a NutshellOptional ItemsOther Quantifiers: RepetitionParentheses and BackreferencesThe Great EscapeExpanding the FoundationLinguistic DiversificationThe Goal of a Regular ExpressionA Few More ExamplesRegular Expression NomenclatureImproving on the Status QuoSummaryPersonal Glimpses
About the ExamplesA Short Introduction to PerlMatching Text with Regular ExpressionsToward a More Real-World ExampleSide Effects of a Successful MatchIntertwined Regular ExpressionsIntermissionModifying Text with Regular ExpressionsExample: Form LetterExample: Prettifying a Stock PriceAutomated EditingA Small Mail UtilityAdding Commas to a Number with LookaroundText-to-HTML ConversionThat Doubled-Word Thing
A Casual Stroll Across the Regex LandscapeThe Origins of Regular ExpressionsAt a GlanceCare and Handling of Regular ExpressionsIntegrated HandlingProcedural and Object-Oriented HandlingA Search-and-Replace ExampleSearch and Replace in Other LanguagesCare and Handling: SummaryStrings, Character Encodings, and ModesStrings as Regular ExpressionsCharacter-Encoding IssuesUnicodeRegex Modes and Match ModesCommon Metacharacters and FeaturesCharacter RepresentationsCharacter Classes and Class-Like ConstructsAnchors and Other “Zero-Width Assertions”Comments and Mode ModifiersGrouping, Capturing, Conditionals, and ControlGuide to the Advanced Chapters
Start Your Engines!Two Kinds of EnginesNew StandardsRegex Engine TypesFrom the Department of Redundancy DepartmentTesting the Engine TypeMatch BasicsAbout the ExamplesRule 1: The Match That Begins Earliest WinsEngine Pieces and PartsRule 2: The Standard Quantifiers Are GreedyRegex-Directed Versus Text-DirectedNFA Engine: Regex-DirectedDFA Engine: Text-DirectedFirst Thoughts: NFA and DFA in ComparisonBacktrackingA Really Crummy AnalogyTwo Important Points on BacktrackingSaved StatesBacktracking and GreedinessMore About Greediness and BacktrackingProblems of GreedinessMulti-Character “Quotes”Using Lazy QuantifiersGreediness and Laziness Always Favor a MatchThe Essence of Greediness, Laziness, and BacktrackingPossessive Quantifiers and Atomic GroupingPossessive Quantifiers, ?+, *+, ++, and {m,n}+The Backtracking of LookaroundIs Alternation Greedy?Taking Advantage of Ordered AlternationNFA, DFA, and POSIX“The Longest-Leftmost”POSIX and the Longest-Leftmost RuleSpeed and EfficiencySummary: NFA and DFA in ComparisonSummary

Regex Balancing ActA Few Short ExamplesContinuing with Continuation LinesMatching an IP AddressWorking with FilenamesMatching Balanced Sets of ParenthesesWatching Out for Unwanted MatchesMatching Delimited TextKnowing Your Data and Making AssumptionsStripping Leading and Trailing WhitespaceHTML-Related ExamplesMatching an HTML TagMatching an HTML LinkExamining an HTTP URLValidating a HostnamePlucking Out a URL in the Real WorldExtended ExamplesKeeping in Sync with Your DataParsing CSV Files
A Sobering ExampleA Simple Change—Placing Your Best Foot ForwardEfficiency Versus CorrectnessAdvancing Further—Localizing the GreedinessReality CheckA Global View of BacktrackingMore Work for a POSIX NFAWork Required During a Non-MatchBeing More SpecificAlternation Can Be ExpensiveBenchmarkingKnow What You’re MeasuringBenchmarking with PHPBenchmarking with JavaBenchmarking with VB.NETBenchmarking with RubyBenchmarking with PythonBenchmarking with TclCommon OptimizationsNo Free LunchEveryone’s Lunch is DifferentThe Mechanics of Regex ApplicationPre-Application OptimizationsOptimizations with the TransmissionOptimizations of the Regex ItselfTechniques for Faster ExpressionsCommon Sense TechniquesExpose Literal TextExpose AnchorsLazy Versus Greedy: Be SpecificSplit Into Multiple Regular ExpressionsMimic Initial-Character DiscriminationUse Atomic Grouping and Possessive QuantifiersLead the Engine to a MatchUnrolling the LoopMethod 1: Building a Regex From Past ExperiencesThe Real “Unrolling-the-Loop” PatternMethod 2: A Top-Down ViewMethod 3: An Internet HostnameObservationsUsing Atomic Grouping and Possessive QuantifiersShort Unrolling ExamplesUnrolling C CommentsThe Freeflowing RegexA Helping Hand to Guide the MatchA Well-Guided Regex is a Fast RegexWrapupIn Summary: Think!
Regular Expressions as a Language ComponentPerl’s Greatest StrengthPerl’s Greatest WeaknessPerl’s Regex FlavorRegex Operands and Regex LiteralsHow Regex Literals Are ParsedRegex ModifiersRegex-Related PerlismsExpression ContextDynamic Scope and Regex Match EffectsSpecial Variables Modified by a MatchThe qr/···/ Operator and Regex ObjectsBuilding and Using Regex ObjectsViewing Regex ObjectsUsing Regex Objects for EfficiencyThe Match OperatorMatch’s Regex OperandSpecifying the Match Target OperandDifferent Uses of the Match OperatorIterative Matching: Scalar Context, with /gThe Match Operator’s Environmental RelationsThe Substitution OperatorThe Replacement OperandThe /e ModifierContext and Return ValueThe Split OperatorBasic SplitReturning Empty ElementsSplit’s Special Regex OperandsSplit’s Match Operand with Capturing ParenthesesFun with Perl EnhancementsUsing a Dynamic Regex to Match Nested PairsUsing the Embedded-Code ConstructUsing local in an Embedded-Code ConstructA Warning About Embedded Code and my VariablesMatching Nested Constructs with Embedded CodeOverloading Regex LiteralsProblems with Regex-Literal OverloadingMimicking Named Capture
“There’s More Than One Way to Do It”Regex Compilation, the /o Modifier, qr/···/, and EfficiencyUnderstanding the “Pre-Match” CopyThe Study FunctionBenchmarkingRegex Debugging InformationFinal Comments
Java’s Regex FlavorJava Support for \p{···} and \P{···}Unicode Line TerminatorsUsing java.util.regexThe Pattern.compile() FactoryPattern’s matcher methodThe Matcher ObjectApplying the RegexQuerying Match ResultsSimple Search and ReplaceAdvanced Search and ReplaceIn-Place Search and ReplaceThe Matcher’s RegionMethod ChainingMethods for Building a ScannerOther Matcher MethodsOther Pattern MethodsPattern’s split Method, with One ArgumentPattern’s split Method, with Two ArgumentsAdditional ExamplesAdding Width and Height Attributes to Image TagsValidating HTML with Multiple Patterns Per MatcherParsing Comma-Separated Values (CSV) TextJava Version DifferencesDifferences Between 1.4.2 and 1.5.0Differences Between 1.5.0 and 1.6
.NET’s Regex FlavorAdditional Comments on the FlavorUsing .NET Regular ExpressionsRegex QuickstartPackage OverviewCore Object OverviewCore Object DetailsCreating Regex ObjectsUsing Regex ObjectsUsing Match ObjectsUsing Group ObjectsStatic “Convenience” FunctionsRegex CachingSupport FunctionsAdvanced .NETRegex AssembliesMatching Nested ConstructsCapture Objects
PHP’s Regex FlavorThe Preg Function Interface“Pattern” ArgumentsThe Preg Functionspreg_matchpreg_match_allpreg_replacepreg_replace_callbackpreg_splitpreg_greppreg_quote“Missing” Preg Functionspreg_regex_to_patternSyntax-Checking an Unknown Pattern ArgumentSyntax-Checking an Unknown RegexRecursive ExpressionsMatching Text with Nested ParenthesesNo Backtracking Into RecursionMatching a Set of Nested ParenthesesPHP Efficiency IssuesThe S Pattern Modifier: “Study”Extended ExamplesCSV Parsing with PHPChecking Tagged Data for Proper Nesting
Chapter 1Chapter 2Chapter 3Chapter 4Chapter 5Chapter 6Chapter 7Chapter 8Chapter 9Chapter 10

Content preview from Mastering Regular Expressions, 3rd Edition

4The Mechanics of Expression Processing

The previous chapter started with an analogy between cars and regular expressions. The bulk of the chapter discussed features, regex flavors, and other “glossy brochure” issues of regular expressions. This chapter continues with that analogy, talking about the all-important regular-expression engine, and how it goes about its work.

Why would you care how it works? As we’ll see, there are several types of regex engines, and the type most commonly used — the type used by Perl, Tcl, Python, the .NET languages, Ruby, PHP, all Java packages I’ve seen, and more — works in such a way that how you craft your expression can influence whether it can match a particular string, where in the string it matches, and how quickly it finds the match or reports the failure. If these issues are important to you, this chapter is for you.

Start Your Engines!

Let’s see how much I can milk this engine analogy. The whole point of having an engine is so that you can get from Point A to Point B without doing much work. The engine does the work for you so you can relax and enjoy the sound system. The engine’s primary task is to turn the wheels, and how it does that isn’t really a concern of yours. Or is it?

Two Kinds of Engines

Well, what if you had an electric car? They’ve been around for a long time, but they aren’t as common as gas cars because they’re hard to design well. If you had one, though, you would have to remember not to put gas in it. If you had a gasoline ...