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Live Introduction to Regular Expressions in maXbox

Live Introduction to Regular Expressions in maXbox

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  • 1. maXbox Starter 20Start with Regular Expressions1.1 From TRex to RegExRegular expressions are the main way many tools matches’ patterns within strings. For example,finding pieces of text within a larger document, or finding a restriction site within a largersequence. This tutorial illustrates what a RegEx is and what you can do to find, match, compareor replace text of documents or code.As you will see Regular expressions are composed of characters, character classes,metacharacters, quantifiers, and assertions. Hope you did already read Starters 1 till 19 at:’s a Regular Expression?A regular expression (RegEx): describes a search pattern of text typically made up from specialcharacters called metacharacters: • You can test whether a string matches the expression pattern • You can use a RegEx to search/replace characters in a string • It’s very powerful, but tough to readRegular expressions occur in many places and masks, not alone in code or languagesenvironments:Text editors, shells or search tools like Grep1 allow also RegEx in search/replace functions, likethe following screen-shot figure out. 1: Find with RegExLet’s jump to history and the beginning of RegEx with Perl. Perl was a horribly flawed and veryuseful scripting language, based on UNIX shell scripting and C, that helped lead to many other1 Global Regular Expression Print / Parser
  • 2. better languages. Perl was and is also excellent for string/file/text processing because it builtregular expressions directly into the language as a first-class data type. Many command-line shell Linux/Mac tools (ed, vi, grep, egrep, sed, awk) support RegularExpressions, for e.g. Grep: grep -e "[pP]hone.*206[0-9]{7}" contacts.txt>> phone { 206-685-2181}Grep is a tool that originated from the UNIX world during the 1970s. It can search through filesand folders (directories in UNIX) and check which lines in those files match a given regularexpression. Grep will output the filenames and the line numbers or the actual lines that matchedthe regular expression.1.2 RegEx out of the BoxAs you already know the tool is split up into the toolbar across the top, the editor or code part inthe centre and the output window at the bottom or the interface part on the right. Change that inthe menu /View at our own style. In maXbox you will execute the RegEx as a script, libraries and units are already built. Before this starter code will work you will need to download maXbox from the website. It canbe down-loaded from (you’ll find the on the top left of the page). Once the download has finished, unzip the file, makingsure that you preserve the folder structure as it is. If you double-click maxbox3.exe the boxopens a default demo program. Test it with F9 / F2 or press Compile and you should hear asound. So far so good now we’ll open the example:309_regex_powertester2.txtIf you can’t find the two files try also the zip-file (48100 bytes) loaded from: let’s take a look at the code of this fist part project. Our first line is01 program RegEx_Power_Tester_TRex;We name it, means the program’s name is above. This example requires two objects from the classes: TRegExpr and TPerlRegEx ofPerlRegEx so the second one is from the well known PCRE Lib.TPerlRegEx is a Delphi VCL wrapper around the open source PCRE library, which implementsPerl-Compatible Regular Expressions.This version of TPerlRegEx is compatible with the TPerlRegEx class (PCRE 7.9) in theRegularExpressionsCore unit in Delphi XE. In fact, the unit in Delphi XE and maXbox3 isderived from the version of TPerlRegEx that we are using now.Let’s do a first RegEx now. We want to check if a name is a valid Pascal name like a syntaxchecker does. We use straight forward a function in the box:732 if ExecRegExpr(^[a-zA-Z_][a-zA-Z0-9_].*,pascal_name_kon) then writeln(pascal name valid) else writeln(pascal name invalid);This is a useful global function:function ExecRegExpr (const ARegExpr, AInputStr: string): boolean; 2
  • 3. It is true if a string AInputString matches regular expression ARegExpr and it will raise anexception if syntax errors in ARegExpr are done.Now let’s analyse our first RegEx step by step ^[a-zA-Z_][a-zA-Z0-9_].*:^ matches the beginning of a line; $ the end[a-z] matches all twenty six small characters from a to z[a-zA-Z_] matches any letter with underscore[a-zA-Z0-9_] matches any letter or digit with underscore.* (a dot) matches any character except n.* * means 0 or more occurrences[ ] group characters into a character set;A lot of rules for the beginning, and they look ugly for novices, but really they are very simple(well, usually simple ;)), handy and powerful tool too. You can validate e-mail addresses; extractphone numbers or ZIP codes from web-pages or documents, search for complex patterns in logfiles and all you can imagine! Rules (templates) can be changed without your programrecompilation! This can be especially useful for user input validation in DBMS and web projects.Try the next one:if ExecRegExpr(M[ae][iy]e?r.*[be], Mairhuberu) then writeln(regex maierhuber true) else writeln(regex maierhuber false);? Means 0 or 1 occurrencesAny item of a regular expression may be followed by another type of metacharacters – callediterators. Using this characters you can specify number of occurrences of previous characters soinside [], most modifier keys act as normal characters:/what[.!*?]*/ matches "what", "what.", "what!", "what?**!", ..So a character class is a way of matching 1 character in the string being searched to any of anumber of characters in the search pattern.Character classes are defined using square brackets. Thus [135] matches any of 1, 3, or 5. Arange of characters (based on ASCII order) can be used in a character class: [0-7] matches anydigit between 0 and 7, and [a-z] matches and small (but not capital) letter. Note that the hyphen is being used as a metacharacter here. To match a literal hyphen in acharacter class, it needs to be the first character. So [-135] matches any of -, 1, 3, or 5. [-0-9]matches any digit or the hyphen.What if we want to define a certain place? An assertion is a statement about the position of thematch pattern within a string. The most common assertions are “^”, which signifies the beginningof a string, and “$”, which signifies the end of the string.For example search all empty or blank lines: Search empty lines: ‘^$’This is how we can assert a valid port number with ^ and $:745 if ExecRegExpr(^(:dd?d?d?d?)$,:80009) then writeln(regex port true) else writeln(regex port false);There are 3 main operators that use regular expressions: 1. Matching (which returns TRUE if a match is found and FALSE if no match is found. 2. Substitution, which substitutes one pattern of characters for another within a string 3. Split, which separates a string into a series of substrings 3
  • 4. If you want to match a certain number of repeats of a group of characters, you can group thecharacters within parentheses. For example, /(cat){3}/ matches 3 reps of “cat” in a row:“catcatcat”. However, /cat{3}/ matches “ca” followed by 3 t’s: “cattt”.And things go on. To negate or reject a character class, that is, to match any character EXCEPTwhat is in the class, use the caret ^ as the first symbol in the class. [^0-9] matches anycharacter that isn’t a digit. [^-0-9] ,matches any character that isn’t a hyphen or a digit.Now its time to reflect: RE Metacharacter Matches… ^ beginning of line $ end of line char Escape the meaning of char following it [^] One character not in the set < Beginning of word anchor > End of word anchor ( ) or ( ) Tags matched characters to be used later (max = 9) | or | Or grouping x{m} Repetition of character x, m times (x,m = integer) x{m,} Repetition of character x, at least m times x{m,n} Repetition of character x between m and m times 2. Overview of MatchesYou can specify a series of alternatives for a pattern using "| to separate them, so that fee|fie|foewill match any of "fee, "fie, or "foe in the target string (as would f(e|i|o)e). The first alternativeincludes everything from the last pattern delimiter ("(, "[, or the beginning of the pattern)up to the first "|, and the last alternative contains everything from the last "| to the next patterndelimiter.For this reason, its common practice to include alternatives in parentheses, to minimizeconfusion about where they start and end.Next a few examples to see the atoms: rex:= (no)+.*; //Print all lines containing one or more consecutiveoccurrences of the pattern “no”. rex:= .*S(h|u).*; //Print all lines containing the uppercase letter “S”,followed by either “h” or “u”. rex:= .*.[^0][^0].*; //Print all lines ending with a period and exactly twonon-zero numbers. 4
  • 5. rex:= .*[0-9]{6}..*; //all lines at least 6 consecutive numbers follow by aperiod.Next we want to see how the objects in the box work:The static versions of the methods are provided for convenience, and should only be used for oneoff matches, if you are matching in a loop or repeating the same search often then you shouldcreate an instance of the TRegEx record and use the non static methods.The RegEx unit defines TRegEx and TMatch as records. That way you don’t have to explicitlycreate and destroy them. Internally, TRegEx uses TPerlRegEx to do the heavy lifting.TPerlRegEx is a class that needs to be created and destroyed like any other class. If you look atthe TRegEx source code, you’ll notice that it uses an interface to destroy the TPerlRegExinstance when TRegEx goes out of scope. Interfaces are reference counted in Delphi, makingthem usable for automatic memory management. The XE interface to PCRE is a layer of units based on contributions from various people, thePCRE API header translations in RegularExpressionsAPI.pas 3: The two Classes in compareClients are seen in picture 3 as the two classes do the same task. After creating the object RegExwe set the options:These are the most important options you can specify: • preCaseLess Tries to match the regex without paying attention to case. If set, Bye will match Bye, bye, BYE and even byE, bYe, etc. Otherwise, only Bye will be matched. Equivalent to Perls /i modifier. • preMultiLine The ^ (beginning of string) and $ (ending of string) regex operaters will also match right after and right before a newline in the Subject string. This effectively treats one string with multiple lines as multiple strings. Equivalent to Perls /m modifier. 5
  • 6. • preSingleLine Normally, dot (.) matches anything but a newline (n). With preSingleLine, dot (.) will match anything, including newlines. This allows a multiline string to be regarded as a single entity. Equivalent to Perls /s modifier. • Note that preMultiLine and preSingleLine can be used together. • preExtended Allow regex to contain extra whitespace, newlines and Perl-style comments, all of which will be filtered out. This is sometimes called "free-spacing mode". • preAnchored Allows the RegEx to match only at the start of the subject or right after the previous match. • preUngreedy Repeat operators (?, *, +, {num,num}) are greedy by default, i.e. they try to match as many characters as possible. Set preUngreedy to use ungreedy repeat operators by default, i.e. they try to match as few characters as possible. Greedy is a strange operator or option. A slight explanation about "greediness". "Greedy" takes as many as possible; "non-greedy" takes as few as possible. For example, b+and b* applied to string abbbbc return bbbb, b+? returns b, b*? returns an empty string,b{2,3}? returns bb, b{2,3} returns bbb.The regular expression engine does “greedy” matching by default!A typical RegEx client session looks like this:848 with TPerlRegEx.Create do try //Perl Delphi RegEx849 RegEx:= <title>(.+?)</title>;850 Options:= [preCaseLess];851 Subject:= testa <title> My TRex on Regex</title> testb;852 If Match then853 ShowMessageBig(Groups[1]) //SubExpressions[1]) ,MatchedText854 else855 ShowMessageBig(Regex Not found);856 finally857 Free;858 end;Subject is the RegEx and the string on which Match will try to match RegEx. Match attempts tomatch the regular expression specified in the RegEx property on the string specified in theSubject property. If Compile has not yet been called, Match will do so for you.Call MatchAgain to attempt to match the RegEx on the remainder of the subject string after asuccessful call to Match. Compile: Before it can be used, the regular expression needs to be compiled. Match will callCompile automatically if you did not do so. If the regular expression will be applied in time-criticalcode, you may wish to compile it during your applications initialization. You may also want to callStudy to further optimize the execution of the RegEx.Let’s have a look at the RegEx itself and the magic behind:<title>(.+?)</title>What’s about this <title>, it must be a global identifier to find or still exists.+? one or more ("non-greedy"), similar to {1,}?It captures everything between the first <title> and the first </title> that follows. This isusually what you want to do with large sequences in a group. 6
  • 7. Greedy names: Greedy matching can cause problems with the use of quantifiers. Imaginethat you have a long DNA sequence and you try to match /ATG(.*)TAG/. The “.*” matches 0 ormore of any character. Greedy matching causes this to take the entire sequence between the firstATG and the last TAG. This could be a very long matched sequence. Note that Regular expressions don’t work very well with nested delimiters or other tree-likedata structures, such as are found in an HTML table or an XML document. We will discussalternatives later in a course. So far we have learned little about RegEx and the past with a TRex eating words as a book output to us;-). Now it’s time to run your program at first with F9 (if you haven’t done yet) and learn something about the 309_regex_powertester2.txt with many code snippets to explore.One of them is a song finder in the appendix to get a song list from an mp3 file and play them! 4: MasteringThere are plenty more regular expression tricks and operations, which can be found inProgramming Perl, or, for the truly devoted, Mastering Regular Expressions.Next we enter part two of the insider information about the implementation. 5: Enter a RegEx building1.3 RegEx in Delphi and maXboxThe TPerlRegEx class aimes at providing any Delphi, Java or C++Builder developer with thesame, powerful regular expression capabilities provided by the Perl programming languagecommunity, created by Larry Wall.It is implemented as a wrapper around the open source PCRE library.The regular expression engine in Delphi XE is PCRE (Perl Compatible Regular Expression). Its afast and compliant (with generally accepted RegEx syntax) engine which has been around for 7
  • 8. many years. Users of earlier versions of Delphi can use it with TPerlRegEx, a Delphi classwrapper around it.TRegEx is a record for convenience with a bunch of methods and static class methods formatching with regular expressions.Ive always used the RegularExpressionsCore unit rather than the higher level stuff becausethe core unit is compatible with the unit that Jan Goyvaerts has provided for free for years. Thatwas my introduction to regular expressions. So I forgot about the other unit. I guess theres eithera bug or it just doesnt work the way one might expect.For new code written in Delphi XE, you should definitely use the RegEx unit that is part of Delphirather than one of the many 3rd party units that may be available. But if youre dealing with UTF-8data, use the RegularExpressionsCore unit to avoid needless UTF-8 to UTF-16 to UTF-8conversions. The procedure Study procedure Study;Allows studying the RegEx. Studying takes time, but will make the execution of the RegEx a lotfaster. Call study if you will be using the same RegEx many times. Study will also call Compileif this had not yet been done.Depending on what the user entered in Edit1 and Memo1, RegEx might end up being a prettycomplicated regular expression that will be applied to the memo text a great many times. Thismakes it worthwhile to spend a little extra time studying the regular expression (later on more).By the way there’s another tool: Compose and analyze RegEx patterns with RegexBuddys easy-to-grasp RegEx blocks and intuitive RegEx tree, instead of or in combination with the traditionalRegEx syntax. Developed by the author of the website, RegexBuddy makes learning and using regular expressions easier thanever. 6: The RegexBuddy and the GUIConclusion: A regular expression (RegEx or regexp for short) is a special text string for describinga search pattern. You can think of regular expressions as wildcards on steroids. You are probablyfamiliar with wildcard notations such as *.txt to find all text files in a file manager. The RegExequivalent is .*.txt$. 8
  • 9. 7: The Secret behind this Regular Expression!?Study method example:Depending on what the user entered in Edit1 and Memo1, RegEx might end up being a prettycomplicated regular expression that will be applied to the memo text a great many times. Thismakes it worthwhile to spend a little extra time studying the regular expression.31 with PerlRegEx1 do begin RegEx:= Edit1.Text; Study; Subject:= Memo1.Lines.Text; Replacement:= Edit2.Text; ReplaceAll; Memo1.Lines.Text:= Subject;end; Try reformat phone numbers from 206-685-2181 format to (206) 685.2181 format to getdata back:You can use back-references when replacing text. Text "captured" in () is given an internalnumber; use number to refer to it elsewhere in the pattern 0 is the overall pattern, 1 is the firstparenthetical capture, 2 the second, ...Example: "A" surrounded by same character: /(.)A1/Example: to swap a last name with a first name:var name = "Durden, Tyler"; name = name.replace(/(w+),s+(w+)/, "$2 $1"); // "Tyler Durden") 9
  • 10. Time of day: For example. 11:30. [01][0-9]:[0-5][0-9] wont work well, because itwould allow such impossible times as 19:00 and 00:30. A more complicated construction worksbetter: (1[012] | [1-9]) :[0-5][0-9]. That is, a 1 followed by 0, 1, or 2, OR any digit 1-9.Feedback @max@kleiner.comLiterature:Kleiner et al., Patterns konkret, 2003, Software & SupportLinks of maXbox and RegEx EKON Slide Show: 10
  • 11. 1.4 AppendixEXAMPLE: Mail Finderprocedure delphiRegexMailfinder;begin // Initialize a test string to include some email addresses. This wouldnormally be your eMail. TestString:= <one@server.domain.xy>,; PR:= TPerlRegEx.Create; try PR.RegEx:= b[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,4}b; PR.Options:= PR.Options + [preCaseLess]; PR.Compile; PR.Subject:= TestString; // <-- tell PR where to look for matches if PR.Match then begin WriteLn(PR.MatchedText); // Extract first address while PR.MatchAgain do WriteLn(PR.MatchedText); // Extract subsequent addresses end; finally PR.Free; end; //Readln;end;EXAMPLE: Songfinderwith TRegExpr.Create do try gstr:= Deep Purple; modifierS:= false; //non greedy Expression:= #EXTINF:d{3},+gstr+ - ([^n].*); if Exec(fstr) then Repeat writeln(Format (Songs of +gstr+: %s, [Match[1]])); (*if AnsiCompareText(Match[1], Woman) > 0 then begin closeMP3; PlayMP3(‘..EKON_13_14_15EKON1606_Woman_From_Tokyo.mp3); end;*) Until Not ExecNext; finally Free;end;//************************************ Code Finished****************************** 11
  • 12. 1.5 Appendix RegexBuddy in Action1.6 Appendix String RegEx methods .match(regexp) returns first match for this string against the given regular expression; if global /g flag is used, returns array of all matches .replace(regexp, text) replaces first occurrence of the regular expression with the given text; if global /g flag is used, replaces all occurrences .search(regexp) returns first index where the given regular expression occurs .split(delimiter[,limit]) breaks apart a string into an array of strings using the given regular as the delimiter; returns the array of tokens 12