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Regular Expression

Mahzad Zahedi, profile picture
Mahzad Zahedi

This document provides an introduction and overview of regular expressions (regex). It begins with an introduction to regex and how they are used. It then outlines the table of contents which covers literal characters, character classes, anchors, boundaries, alternation, optional and repetitive items, and grouping. The document discusses topics like literal characters, special characters, character classes, shorthand classes, negated classes, metacharacters inside classes, dot matching, and start/end of string anchors. It provides examples and explanations of how regex engines work to match patterns.

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Mahzad.Zahedi@rcisp.com May 2014 REGULAR EXPRESSIONS)REGEX) 1
REFERENCE The Complete Tutorial (Temp folder) 2
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 3
INTRODUCTION  A regular expression (regex or regexp) is a special text string for describing a search pattern, validate data.  A regular expression “engine” is a piece of software that can process regular expressions.  There are many software applications and programming languages that support regular expressions. 4
.NET 1.0–4.5 Java 4–8 Perl 5.8–5.18 PCRE C library = Perl Compatible Regular Expressions PHP Delphi R JavaScript VBScript XRegExp Python Ruby Tcl ARE POSIX BRE • similar to the one used by the traditional UNIX grep command • most metacharacters require a backslash to give the metacharacter a{1,2} matches a or aa POSIX ERE • similar to the one used by the UNIX egrep command • Quantifiers ?, +, {n}, {n,m} and {n,} • Backreferences • Alternation GNU BRE GNU ERE Oracle XML XPath 5 EditPrO
INTRODUCTION(CONT.)  Advantage:  Reducing development time for Programmer  Fast executing  Ex : reg(ular expressions?|ex(p|es)?)  regular expressions  regular  regexp  regexp  regexes 6
INTRODUCTION(CONT.)  Pattern matching an esentional problem  Many applications need to "parse" a input 1) URLs 2) Log Files: 3) XML http://first.dk/index.php?id=141&view=details 13/02/2010 66.249.65.107 get /support.html 20/02/2010 42.116.32.64 post /search.html protocol host path query-string (list of key-value pairs) <article> <title>Three Models for the...</title> <author>Noam Chomsky</author> <year>1956</year> </article> 7
LITERAL CHARACTERS & SPECIAL CHARACTERS  Literals  A single literal character, ex : «a»  “Jack is a boy”  Some literal characters  Apply «cat» to “He captured a catfish for his cat.”  Non-Printable Characters  «t » tab character (ASCII 0x09)  «r» for carriage return (0x0D)  «n» for line feed (0x0A)  … 1-‫انطباق‬‫ای‬‫ر‬‫ب‬‫تالش‬‫ج‬‫ر‬‫توکن‬ ‫اولین‬‫کس‬ ‫شته‬‫ر‬ ‫با‬:C‫با‬H‫شکست‬ ‫و‬!! 2-‫موجود‬ ‫جکس‬‫ر‬‫از‬ ‫ی‬‫دیگر‬‫جایگشت‬ ‫شته‬‫ر‬ ‫در‬‫بعدی‬‫اکتر‬‫ر‬‫کا‬‫لذا‬‫نیست‬ 3-‫تو‬‫اغ‬‫ر‬‫س‬ ‫موفق‬‫انطباق‬ ‫مین‬‫ر‬‫چها‬ ‫در‬‫کن‬ ‫بعدی‬ 4-‫میخورد‬‫شکست‬‫انطباق‬ ‫ششمین‬‫در‬‫و‬ ‫مین‬‫ر‬‫چها‬ ‫در‬‫میشود‬‫متوجه‬‫موف‬‫ی‬ ‫بررس‬‫ق‬ ‫م‬ ‫ادامه‬ ‫اکتر‬‫ر‬‫کا‬ ‫پنجمین‬‫از‬ ‫و‬ ‫نبودده‬‫ی‬ ‫دهد‬ ∕∕ 8
LITERAL CHARACTERS & SPECIAL CHARACTERS (CONT.)  Reserve certain characters for special use Meaning char Beginning of string ^ caret End of string $ doller sign Any character except newline . dot Match 0 or more * star Match 1 or more + plus Match 0 or 1 ? Question mark alternative | pipe symbol Grouping; ”store” ( ) parenthesis Special backslash opening square bracket [ 9
LITERAL CHARACTERS & SPECIAL CHARACTERS (CONT.)  use any of these characters as a literal in a regex If you forget to escape a special character NOTE !  Most regular expression flavors treat the brace «{» as a literal character, unless it is part of a repetition operator like « M{1,3}».  An exception to this rule is the java.util.regex 1+1=2 literal 123+111=234 other meaning +1=2 ERROR 1+1=2 10
LITERAL CHARACTERS & SPECIAL CHARACTERS (CONT.)  Q...E escape sequence  E.g. «Q*d+*E» matches the literal text „*d+*”. o Special Characters and Programming Languages  Compiler will turn the escaped backslash in the source code into a single backslash in the string that is passed on to the regex library  The regex «1+1=2» as “1+1=2”  The regex «c:temp» as “c:temp” compiler Regex lib 11
FIRST LOOK AT HOW A REGEX ENGINE WORKS INTERNALL  Two kinds of regular expression engines:  text-directed- DFA  regex-directed- NFA  awk, egrep, flex, lex, MySQL are text-directed  A few of versions are regex- directed  The regex directed is more powerful. 12
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 13
CHARACTER CLASSES OR CHARACTER SETS  To match only one out of several characters  «gr[ae]y» = > „ gray” or „grey” (for both American or British English )  Using a hyphen inside a character class to specify a range of characters.  [0-9a-fA-F]  Useful Application  Find a word, even if it is misspelled => «sep[ae]r[ae]te»  Find an identifier => «[A-Za-z_][A-Za-z_0-9]*»  Find a C-style hexadecimal number => «0[xX][A-Fa-f0-9]+» 14
NEGATED CHARACTER CLASSES  [^ …]  Match any character that is not in the character class.  «q[^u]» : “a q followed by a character that is not a u”. => Iraq is a country  Negated character class still must match a character. [^abc] => fdgha  Unlike the dot, negated character classes also match line break characters. 15
METACHARACTERS INSIDE CHARACTER CLASSES  Metacharacters inside a character class are:  The closing bracket ( ] )=>(])  The backslash ()=> []  The caret (^) => [^]  The hyphen (-)=> [-]  [+*]=[+*] -- > reducing readability  Other Solutions:  Placing them in a position where they do not take on their special meaning.  Closing bracket right after the opening bracket []x]  Caret anywhere except right after the opening bracket [x^]  Hyphen any where except middle [-x] 16
METACHARACTERS INSIDE CHARACTER CLASSES  All non-printable characters in character classes just like outside of character classes.  E.g. [$u20AC] : dollar or euro sign  Perl and PCRE also support the Q...E sequence inside character classes  E.g. «[Q[-]E]» matches „[”, „-” or „]”.  POSIX regular expressions treat the backslash as a literal character inside character classes.  Can’t use backslashes to escape  So just use in correct position 17
SHORTHAND CHARACTER CLASSES  Both inside and outside the square brackets are used  Ex: 1 + 2 = 3  sd=whitespace followed by a digit “ 2”  [sd]=whitespace or digit “1 2 3” Class Meaninig w Word character, [a-zA-z0-9_]. d Digit character, [0-9]. s Whitespace character, [ nrt ]. W Non-word character, [^a-zA-z0-9_] =[^w] D Non-digit character, [^0-9]=[^d] S Non-whitespace character, [^ nrft ]=[^s] 18
NEGATED SHORTHAND CHARACTER CLASSES  «[DS]» is not the same as «[^ds]».  [^ds]= any char that is not a digit or whitespace.  123 5] ⌐(a U b )  [DS] =any char that is either not a digit, or is not whitespace.  123 5] ⌐ a U ⌐b 19
REPEATING CHARACTER CLASSES  By using the «?», «*» or «+» operators  «[0-9]+» “833337” „222” …  For repeating the matched character, rather than the class we need “backreferences”  «([0-9])1+»  will match „222”  will match „3333” for “833337 “ 20
LOOKING INSIDE THE REGEX ENGINE  The order inside a character class does not matter  Ex : «gr[ae]y» “Is his hair grey or gray?” 1. Failing to match “g” every 12 steps 2. „g” is matched in 13th step 3. Matching “r” token in the regex with “r” in text 4. Failing to match “a“ token with “e” 5. Try to match other permutations of the regex pattern 6. Matching the last regex token with “y” in text 7. the leftmost match was returned : grey 21
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 22
THE DOT MATCHES (ALMOST) ANY CHARACTER  The most commonly misused metacharacter.  The dot will not match a newline character by default (Why)?  «[^n]» (UNIX regex flavors)  «[^rn]» (Widows regex flavors)  In Perl, the mode where the dot also matches newlines is called "single-line mode“  In .NET framework “Regex.Match("string", "regex", RegexOptions.Singleline)”  JavaScript and VBScript do not have an option to make the dot match line break characters : «[sS]» 23
USE THE DOT SPARINGLY  The dot is a very powerful regex metacharacter  It allows you to be lazy ,Ex: mm/dd/yy format  Solutions: • dd.dd.dd 02512703 • dd[- /.]dd[- /.]dd 99/99/99 • (0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01]) [- .](19|20)dd 09/31/2079 24
USE NEGATED CHARACTER SETS INSTEAD OF THE DOT  star is greedy  Ex: we have a problem with "string one" and "string two "  Regexp : ".*"  "string one" and "string two“  Regexp : "[^"rn]*"  "string one" "string two" 25
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 26
START OF STRING AND END OF STRING ANCHORS  Literals and class characters match a character  Anchors do not match any character at all. Instead, they match a position  Caret «^» : «^a» to “abc”  Dolor sign «$» : «c$» to “abc”  Useful Application  For validating user input, using anchors is very important  if ($input =~ m/d+/) qsdf4ghjk => «^d+$» qsdf4ghjk 44467 27
USING ^ AND $ AS START OF LINE AND END OF LINE ANCHORS  If you have a string consisting of multiple lines,Ex:  “ first linen second line”  In tools as EditPad Pro (work with entire files)  In Programming Languages  Perl : "multi-line mode“  m/^regex$/m 28
PERMANENT START OF STRING AND END OF STRING ANCHORS  «A» : only ever matches at the start of the file  «Z» : only ever matches at the end of the file  Anchors match at a position, rather than matching a character  Anchors can result in a zero-length match.  Since the match does not include any characters, nothing is deleted in replcament  In VB.NET  Dim Quoted as String = Regex.Replace(Original, "^", "> ", RegexOptions.Multiline) 29
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 30
WORD BOUNDARIES  The metacharacter «b» is an anchor like ^“ ” & “$”  This match is zero-length.  Simply put: «b» allows you to perform a “whole words only”  «b4b» matches a “4” 44 a4  2 positions :  Before the first & last word character  Between a word character and a non-word character 31
LOOKING INSIDE THE REGEX ENGINE  Ex: «bisb» string : “This island is beautiful”.  “b” matches position before “T”  Matching the next token: the literal «i»  The engine does not advance to the next character in the string, because the previous regex token was zero-lenght, «i» does not match “T”.  «b» can not match at the position between the “T” and the “h”.  ….  POSIX does not support word boundaries at all. 32
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 33
ALTERNATION WITH THE VERTICAL BAR OR PIPE SYMBOL  Similar to character classes to match a single character  Remember That The Regex Engine Is Eager  It will stop searching as soon as it finds a valid match.  RE: Get|GetValue|Set|SetValue  Str : SetValue What are solutions? 1-‫توکن‬ ‫اولین‬G‫و‬ ‫داده‬‫انطباق‬ ‫ل‬‫او‬ ‫اکتر‬‫ر‬‫کا‬‫با‬ ‫شکست‬!! 2-‫بعدی‬ ‫های‬‫گزینه‬«‫یا‬»‫شکست‬ ‫و‬!! 3-‫بعدی‬‫توکن‬S‫با‬S‫و‬ ‫داده‬ ‫ق‬‫انطبا‬‫شته‬‫ر‬‫در‬ ‫موفق‬!‫تا‬‫ادامه‬ ‫و‬«t» 4-،‫جیکس‬‫ر‬‫بودن‬ ‫مشتاق‬ ‫خاطر‬‫به‬SET ‫میشود‬‫برگردونده‬ 34
ALTERNATION WITH THE VERTICAL BAR OR PIPE SYMBOL (CONT.)  Solutions are:  Changing the order of options  GetValue|Get|SetValue|Set  Using greedy feature of question mark ”?”  Get(Value)?|Set(Value)?  Using b  b(Get|GetValue|Set|SetValue)b  The POSIX standard mandates that the longest match be returned, regardless if the regex an NFA or DFA algorithm. 35
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 36
OPTIONAL ITEMS  “?” makes the preceding token in the regular expression optional.  You can make several tokens optional by grouping them together using round brackets  Feb(ruary)? 23(rd)?  „February 23rd”, „February 23”, „Feb 23rd” , „Feb 23”.  Important Regex Concept: Greediness  The engine will always try to match that part. Only if this causes the entire regular expression to fail, will try ignoring the part the question mark applies to. 37
LOOKING INSIDE THE REGEX ENGINE  EX: «colou?r» , Str: “The colonel likes the color green”. 1. 5th char matches successfully from “c” to “o” 2. Checking wheather “u” matches “n” and fail  Question mark : failing is accesptable. 3. Next token , fails to match “n”. 4. starts again trying to match «c» to the first o in “colonel”. 5. …. 38
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 39
REPETITION WITH STAR AND PLUS  Valid HTML tag  «<[A-Za-z][A-Za-z0-9]*>»  «<[A-Za-z0-9]+>» „<1>” Class Meaninig * 0 or more + 1 or more ? 0 or 1 {3} Exactly 3 {3,} 3 or more {3,5} 3, 4 or 5 Add a ? to a quantifier to make it ungreedy. 40
WATCH OUT FOR THE GREEDINESS!  EX: Matching HTML tag  This is a <EM>first</EM> test  <.+> 1. The first token in the regex is «<». 2. The next token is the dot, which matches any character except newlines 3. The dot is repeated by the plus. The plus is greedy 4. The dot fails when the engine has reached the void after the end of the string. 5. Engine continue with the next token «>» &can not match 6. The engine remembers that the plus has repeated the dot more often than is required so it backtrack 7. It is reduced to „EM>first</EM> tes” and next token in the regex is still «>» 8. It will continue for the first valid match (eager) 41
LAZINESS INSTEAD OF GREEDINESS  Lazy quantifiers are sometimes also called “ungreedy”  This is a <EM>first</EM> test  “<.+?>” 1. «<» matches the first „<” in the string 2. The next token is the dot, this time repeated by a lazy plus  This tells the regex engine to repeat the dot as few times as possible (MIN=1)  Matches “.” With “E” 3. Matches “>” with “M” and fails • But this time, the backtracking will force the lazy plus to expand 4. Return <EM> </EM> 42
LOOKING INSIDE THE REGEX ENGINE  Ex: <([A-Z][A-Z0-9]*)[^>]*>.*?</1>  Str: “Testing <B><I>bold italic</I></B> text” 1. Matching at the first „<” 2. «[A-Z]» matches „B” & advances to «[A-Z0-9]» and “>” 3. This match fails. However, because of the star, that’s perfectly fine 4. Storing what was matched inside them, „B” is stored 5. The regex is advanced to «[^>]» & string remains at “>” & go to 3 6. Matching “>” with “>” 7. The next token is a dot, repeated by a lazy star 43
AN ALTERNATIVE TO LAZINESS  An option for making the plus lazy instead of backtracking  Greedy plus and a negated character class  <EM>first</EM>  «<[^>]+>»  Backtracking slows down the regex engine  you will save plenty of CPU cycles when using such a regex 44
REPEATING Q...E ESCAPE SEQUENCE  «Q*d+*E+»  In Perl : “*d+**d+*”  In java : “*d+**d+*”  If you want Java to return the same match as Perl  «Q*d+E*+»  If you want Perl to repeat the whole sequence like Java does  «(Q*d+*E)+» 45
TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 46
USE ROUND BRACKETS FOR GROUPING  Grouping the part of the regular expression together for applying a regex operator  Creating a Backreference  reuses part of the regex match  slows down the regex engine  Optimize this regular expression into «Set(?:Value)?  How to Use Backreferences  abc5abc  )[abc])+51$ 1=> a b c - > abbc5c  <div>hello</div>  <([a-z]*)>.*</1> 47
REPETITION AND BACKREFERENCES  ([abc]+)» & «([abc])+» to “cab” string  ([abc]+)» : “cab” to be referenced  «([abc])+» : “b” to be referenced 48
USE ROUND BRACKETS FOR GROUPING (CONT.)  Reusing the same backreference more than once.  ([a-c])x1x1» „axaxa” „bxbxb” „cxcxc”  Backreferences Cannot be used inside itself.  ([abc]1)  Round brackets Cannot be used inside character classes, as metacharacters.  (a)[1b]  Useful Example: Checking for Doubled Words  «b(w+)s+1b» 49
POSIX CLASS Posix ASCII ‫توضیح‬ [:alnum:] [A-Za-z0-9] ‫همه‬‫وعددی‬ ‫حرفی‬ ‫های‬‫اکتر‬‫ر‬‫کا‬ [:alpha:] [A-Za-z] ‫و‬‫بزرگ‬ ‫حروف‬‫کوچک‬ [:blank:] [t] ‫فاصله‬‫تب‬ ‫و‬ [:digit:] [0-9] ‫اعداد‬ [:punct:] [?<=>;:/.,+*()' &%$#”!-] @[]^_`{|}~] ‫عالیم‬‫ی‬ ‫ش‬‫ر‬‫نگا‬ 50
ٍEXAMPLE 1  Example 1. Beginning of line ( ^ )  grep "^Nov 10" messages.1  Example 2. End of the line ( $)  grep "terminating.$" messages . Nov 10 01:12:55 gs123 ntpd[2241]: time reset +0.177479 s Nov 10 01:17:17 gs123 ntpd[2241]: synchronized to OCAL(0) Nov 10 01:18:49 gs123 ntpd[2241]: synchronized to 15.1.13.13 Jul 12 17:01:09 cloneme kernel: Kernel log daemon terminating. Oct 28 06:29:54 cloneme kernel: Kernel log daemon terminating 51
EXAMPLE 2  Example 3. quantifier (*)(+)(?)  [hc]*at =cchat,hcat,hhhat,at 0 or more  [hc]+at= ccchat, hcat, No at 1 or more  [hc]?at= hat, cat, at 0 or 1  Example 4.Escaping the special character ()  grep "127.0.0.1" /var/log/messages.4 Oct 28 06:31:10 btovm871 ntpd[2241]: Listening on interface lo, 127.0.0.1#123 Enabled 52
ٍEXAMPLE 3  Example 5.Excluding specific characters ‫الف‬:  Match text hog  Match text dog ---- > [^b]og  Skip Text bog ‫ب‬:‫غیر‬ ‫ی‬‫اکتر‬‫ر‬‫کا‬a‫یا‬b‫یا‬c [^abc] abccc adb gh 53
EXAMPLE 4  Example 6. Composite syntax ‫است‬ ‫یر‬‫ز‬ ‫شرح‬ ‫به‬ ‫الگ‬‫اطالعات‬:  Sun Jun 4 22:08:39 2006 [pid 21611] [dcid] OK LOGIN: Client “192.168.1.1”  ^w+sw+sd+ S+ d+ [pid d+]s [(w+)] OK LOGIN: Client “(d+.d+.d+.d+)”$ 54

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Regular Expression

  • 3. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 3
  • 4. INTRODUCTION  A regular expression (regex or regexp) is a special text string for describing a search pattern, validate data.  A regular expression “engine” is a piece of software that can process regular expressions.  There are many software applications and programming languages that support regular expressions. 4
  • 5. .NET 1.0–4.5 Java 4–8 Perl 5.8–5.18 PCRE C library = Perl Compatible Regular Expressions PHP Delphi R JavaScript VBScript XRegExp Python Ruby Tcl ARE POSIX BRE • similar to the one used by the traditional UNIX grep command • most metacharacters require a backslash to give the metacharacter a{1,2} matches a or aa POSIX ERE • similar to the one used by the UNIX egrep command • Quantifiers ?, +, {n}, {n,m} and {n,} • Backreferences • Alternation GNU BRE GNU ERE Oracle XML XPath 5 EditPrO
  • 6. INTRODUCTION(CONT.)  Advantage:  Reducing development time for Programmer  Fast executing  Ex : reg(ular expressions?|ex(p|es)?)  regular expressions  regular  regexp  regexp  regexes 6
  • 7. INTRODUCTION(CONT.)  Pattern matching an esentional problem  Many applications need to "parse" a input 1) URLs 2) Log Files: 3) XML http://first.dk/index.php?id=141&view=details 13/02/2010 66.249.65.107 get /support.html 20/02/2010 42.116.32.64 post /search.html protocol host path query-string (list of key-value pairs) <article> <title>Three Models for the...</title> <author>Noam Chomsky</author> <year>1956</year> </article> 7
  • 8. LITERAL CHARACTERS & SPECIAL CHARACTERS  Literals  A single literal character, ex : «a»  “Jack is a boy”  Some literal characters  Apply «cat» to “He captured a catfish for his cat.”  Non-Printable Characters  «t » tab character (ASCII 0x09)  «r» for carriage return (0x0D)  «n» for line feed (0x0A)  … 1-‫انطباق‬‫ای‬‫ر‬‫ب‬‫تالش‬‫ج‬‫ر‬‫توکن‬ ‫اولین‬‫کس‬ ‫شته‬‫ر‬ ‫با‬:C‫با‬H‫شکست‬ ‫و‬!! 2-‫موجود‬ ‫جکس‬‫ر‬‫از‬ ‫ی‬‫دیگر‬‫جایگشت‬ ‫شته‬‫ر‬ ‫در‬‫بعدی‬‫اکتر‬‫ر‬‫کا‬‫لذا‬‫نیست‬ 3-‫تو‬‫اغ‬‫ر‬‫س‬ ‫موفق‬‫انطباق‬ ‫مین‬‫ر‬‫چها‬ ‫در‬‫کن‬ ‫بعدی‬ 4-‫میخورد‬‫شکست‬‫انطباق‬ ‫ششمین‬‫در‬‫و‬ ‫مین‬‫ر‬‫چها‬ ‫در‬‫میشود‬‫متوجه‬‫موف‬‫ی‬ ‫بررس‬‫ق‬ ‫م‬ ‫ادامه‬ ‫اکتر‬‫ر‬‫کا‬ ‫پنجمین‬‫از‬ ‫و‬ ‫نبودده‬‫ی‬ ‫دهد‬ ∕∕ 8
  • 9. LITERAL CHARACTERS & SPECIAL CHARACTERS (CONT.)  Reserve certain characters for special use Meaning char Beginning of string ^ caret End of string $ doller sign Any character except newline . dot Match 0 or more * star Match 1 or more + plus Match 0 or 1 ? Question mark alternative | pipe symbol Grouping; ”store” ( ) parenthesis Special backslash opening square bracket [ 9
  • 10. LITERAL CHARACTERS & SPECIAL CHARACTERS (CONT.)  use any of these characters as a literal in a regex If you forget to escape a special character NOTE !  Most regular expression flavors treat the brace «{» as a literal character, unless it is part of a repetition operator like « M{1,3}».  An exception to this rule is the java.util.regex 1+1=2 literal 123+111=234 other meaning +1=2 ERROR 1+1=2 10
  • 11. LITERAL CHARACTERS & SPECIAL CHARACTERS (CONT.)  Q...E escape sequence  E.g. «Q*d+*E» matches the literal text „*d+*”. o Special Characters and Programming Languages  Compiler will turn the escaped backslash in the source code into a single backslash in the string that is passed on to the regex library  The regex «1+1=2» as “1+1=2”  The regex «c:temp» as “c:temp” compiler Regex lib 11
  • 12. FIRST LOOK AT HOW A REGEX ENGINE WORKS INTERNALL  Two kinds of regular expression engines:  text-directed- DFA  regex-directed- NFA  awk, egrep, flex, lex, MySQL are text-directed  A few of versions are regex- directed  The regex directed is more powerful. 12
  • 13. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 13
  • 14. CHARACTER CLASSES OR CHARACTER SETS  To match only one out of several characters  «gr[ae]y» = > „ gray” or „grey” (for both American or British English )  Using a hyphen inside a character class to specify a range of characters.  [0-9a-fA-F]  Useful Application  Find a word, even if it is misspelled => «sep[ae]r[ae]te»  Find an identifier => «[A-Za-z_][A-Za-z_0-9]*»  Find a C-style hexadecimal number => «0[xX][A-Fa-f0-9]+» 14
  • 15. NEGATED CHARACTER CLASSES  [^ …]  Match any character that is not in the character class.  «q[^u]» : “a q followed by a character that is not a u”. => Iraq is a country  Negated character class still must match a character. [^abc] => fdgha  Unlike the dot, negated character classes also match line break characters. 15
  • 16. METACHARACTERS INSIDE CHARACTER CLASSES  Metacharacters inside a character class are:  The closing bracket ( ] )=>(])  The backslash ()=> []  The caret (^) => [^]  The hyphen (-)=> [-]  [+*]=[+*] -- > reducing readability  Other Solutions:  Placing them in a position where they do not take on their special meaning.  Closing bracket right after the opening bracket []x]  Caret anywhere except right after the opening bracket [x^]  Hyphen any where except middle [-x] 16
  • 17. METACHARACTERS INSIDE CHARACTER CLASSES  All non-printable characters in character classes just like outside of character classes.  E.g. [$u20AC] : dollar or euro sign  Perl and PCRE also support the Q...E sequence inside character classes  E.g. «[Q[-]E]» matches „[”, „-” or „]”.  POSIX regular expressions treat the backslash as a literal character inside character classes.  Can’t use backslashes to escape  So just use in correct position 17
  • 18. SHORTHAND CHARACTER CLASSES  Both inside and outside the square brackets are used  Ex: 1 + 2 = 3  sd=whitespace followed by a digit “ 2”  [sd]=whitespace or digit “1 2 3” Class Meaninig w Word character, [a-zA-z0-9_]. d Digit character, [0-9]. s Whitespace character, [ nrt ]. W Non-word character, [^a-zA-z0-9_] =[^w] D Non-digit character, [^0-9]=[^d] S Non-whitespace character, [^ nrft ]=[^s] 18
  • 19. NEGATED SHORTHAND CHARACTER CLASSES  «[DS]» is not the same as «[^ds]».  [^ds]= any char that is not a digit or whitespace.  123 5] ⌐(a U b )  [DS] =any char that is either not a digit, or is not whitespace.  123 5] ⌐ a U ⌐b 19
  • 20. REPEATING CHARACTER CLASSES  By using the «?», «*» or «+» operators  «[0-9]+» “833337” „222” …  For repeating the matched character, rather than the class we need “backreferences”  «([0-9])1+»  will match „222”  will match „3333” for “833337 “ 20
  • 21. LOOKING INSIDE THE REGEX ENGINE  The order inside a character class does not matter  Ex : «gr[ae]y» “Is his hair grey or gray?” 1. Failing to match “g” every 12 steps 2. „g” is matched in 13th step 3. Matching “r” token in the regex with “r” in text 4. Failing to match “a“ token with “e” 5. Try to match other permutations of the regex pattern 6. Matching the last regex token with “y” in text 7. the leftmost match was returned : grey 21
  • 22. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 22
  • 23. THE DOT MATCHES (ALMOST) ANY CHARACTER  The most commonly misused metacharacter.  The dot will not match a newline character by default (Why)?  «[^n]» (UNIX regex flavors)  «[^rn]» (Widows regex flavors)  In Perl, the mode where the dot also matches newlines is called "single-line mode“  In .NET framework “Regex.Match("string", "regex", RegexOptions.Singleline)”  JavaScript and VBScript do not have an option to make the dot match line break characters : «[sS]» 23
  • 24. USE THE DOT SPARINGLY  The dot is a very powerful regex metacharacter  It allows you to be lazy ,Ex: mm/dd/yy format  Solutions: • dd.dd.dd 02512703 • dd[- /.]dd[- /.]dd 99/99/99 • (0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01]) [- .](19|20)dd 09/31/2079 24
  • 25. USE NEGATED CHARACTER SETS INSTEAD OF THE DOT  star is greedy  Ex: we have a problem with "string one" and "string two "  Regexp : ".*"  "string one" and "string two“  Regexp : "[^"rn]*"  "string one" "string two" 25
  • 26. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 26
  • 27. START OF STRING AND END OF STRING ANCHORS  Literals and class characters match a character  Anchors do not match any character at all. Instead, they match a position  Caret «^» : «^a» to “abc”  Dolor sign «$» : «c$» to “abc”  Useful Application  For validating user input, using anchors is very important  if ($input =~ m/d+/) qsdf4ghjk => «^d+$» qsdf4ghjk 44467 27
  • 28. USING ^ AND $ AS START OF LINE AND END OF LINE ANCHORS  If you have a string consisting of multiple lines,Ex:  “ first linen second line”  In tools as EditPad Pro (work with entire files)  In Programming Languages  Perl : "multi-line mode“  m/^regex$/m 28
  • 29. PERMANENT START OF STRING AND END OF STRING ANCHORS  «A» : only ever matches at the start of the file  «Z» : only ever matches at the end of the file  Anchors match at a position, rather than matching a character  Anchors can result in a zero-length match.  Since the match does not include any characters, nothing is deleted in replcament  In VB.NET  Dim Quoted as String = Regex.Replace(Original, "^", "> ", RegexOptions.Multiline) 29
  • 30. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 30
  • 31. WORD BOUNDARIES  The metacharacter «b» is an anchor like ^“ ” & “$”  This match is zero-length.  Simply put: «b» allows you to perform a “whole words only”  «b4b» matches a “4” 44 a4  2 positions :  Before the first & last word character  Between a word character and a non-word character 31
  • 32. LOOKING INSIDE THE REGEX ENGINE  Ex: «bisb» string : “This island is beautiful”.  “b” matches position before “T”  Matching the next token: the literal «i»  The engine does not advance to the next character in the string, because the previous regex token was zero-lenght, «i» does not match “T”.  «b» can not match at the position between the “T” and the “h”.  ….  POSIX does not support word boundaries at all. 32
  • 33. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 33
  • 34. ALTERNATION WITH THE VERTICAL BAR OR PIPE SYMBOL  Similar to character classes to match a single character  Remember That The Regex Engine Is Eager  It will stop searching as soon as it finds a valid match.  RE: Get|GetValue|Set|SetValue  Str : SetValue What are solutions? 1-‫توکن‬ ‫اولین‬G‫و‬ ‫داده‬‫انطباق‬ ‫ل‬‫او‬ ‫اکتر‬‫ر‬‫کا‬‫با‬ ‫شکست‬!! 2-‫بعدی‬ ‫های‬‫گزینه‬«‫یا‬»‫شکست‬ ‫و‬!! 3-‫بعدی‬‫توکن‬S‫با‬S‫و‬ ‫داده‬ ‫ق‬‫انطبا‬‫شته‬‫ر‬‫در‬ ‫موفق‬!‫تا‬‫ادامه‬ ‫و‬«t» 4-،‫جیکس‬‫ر‬‫بودن‬ ‫مشتاق‬ ‫خاطر‬‫به‬SET ‫میشود‬‫برگردونده‬ 34
  • 35. ALTERNATION WITH THE VERTICAL BAR OR PIPE SYMBOL (CONT.)  Solutions are:  Changing the order of options  GetValue|Get|SetValue|Set  Using greedy feature of question mark ”?”  Get(Value)?|Set(Value)?  Using b  b(Get|GetValue|Set|SetValue)b  The POSIX standard mandates that the longest match be returned, regardless if the regex an NFA or DFA algorithm. 35
  • 36. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 36
  • 37. OPTIONAL ITEMS  “?” makes the preceding token in the regular expression optional.  You can make several tokens optional by grouping them together using round brackets  Feb(ruary)? 23(rd)?  „February 23rd”, „February 23”, „Feb 23rd” , „Feb 23”.  Important Regex Concept: Greediness  The engine will always try to match that part. Only if this causes the entire regular expression to fail, will try ignoring the part the question mark applies to. 37
  • 38. LOOKING INSIDE THE REGEX ENGINE  EX: «colou?r» , Str: “The colonel likes the color green”. 1. 5th char matches successfully from “c” to “o” 2. Checking wheather “u” matches “n” and fail  Question mark : failing is accesptable. 3. Next token , fails to match “n”. 4. starts again trying to match «c» to the first o in “colonel”. 5. …. 38
  • 39. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 39
  • 40. REPETITION WITH STAR AND PLUS  Valid HTML tag  «<[A-Za-z][A-Za-z0-9]*>»  «<[A-Za-z0-9]+>» „<1>” Class Meaninig * 0 or more + 1 or more ? 0 or 1 {3} Exactly 3 {3,} 3 or more {3,5} 3, 4 or 5 Add a ? to a quantifier to make it ungreedy. 40
  • 41. WATCH OUT FOR THE GREEDINESS!  EX: Matching HTML tag  This is a <EM>first</EM> test  <.+> 1. The first token in the regex is «<». 2. The next token is the dot, which matches any character except newlines 3. The dot is repeated by the plus. The plus is greedy 4. The dot fails when the engine has reached the void after the end of the string. 5. Engine continue with the next token «>» &can not match 6. The engine remembers that the plus has repeated the dot more often than is required so it backtrack 7. It is reduced to „EM>first</EM> tes” and next token in the regex is still «>» 8. It will continue for the first valid match (eager) 41
  • 42. LAZINESS INSTEAD OF GREEDINESS  Lazy quantifiers are sometimes also called “ungreedy”  This is a <EM>first</EM> test  “<.+?>” 1. «<» matches the first „<” in the string 2. The next token is the dot, this time repeated by a lazy plus  This tells the regex engine to repeat the dot as few times as possible (MIN=1)  Matches “.” With “E” 3. Matches “>” with “M” and fails • But this time, the backtracking will force the lazy plus to expand 4. Return <EM> </EM> 42
  • 43. LOOKING INSIDE THE REGEX ENGINE  Ex: <([A-Z][A-Z0-9]*)[^>]*>.*?</1>  Str: “Testing <B><I>bold italic</I></B> text” 1. Matching at the first „<” 2. «[A-Z]» matches „B” & advances to «[A-Z0-9]» and “>” 3. This match fails. However, because of the star, that’s perfectly fine 4. Storing what was matched inside them, „B” is stored 5. The regex is advanced to «[^>]» & string remains at “>” & go to 3 6. Matching “>” with “>” 7. The next token is a dot, repeated by a lazy star 43
  • 44. AN ALTERNATIVE TO LAZINESS  An option for making the plus lazy instead of backtracking  Greedy plus and a negated character class  <EM>first</EM>  «<[^>]+>»  Backtracking slows down the regex engine  you will save plenty of CPU cycles when using such a regex 44
  • 45. REPEATING Q...E ESCAPE SEQUENCE  «Q*d+*E+»  In Perl : “*d+**d+*”  In java : “*d+**d+*”  If you want Java to return the same match as Perl  «Q*d+E*+»  If you want Perl to repeat the whole sequence like Java does  «(Q*d+*E)+» 45
  • 46. TABLE OF CONTENT  1. Introduction  2. Literal Characters  3. First Look at How a Regex Engine Works  4. Character Classes or Character Sets  5. The Dot Matches (Almost) Any Character .  6. Start of String and End of String Anchors  7. Word Boundaries  8. Alternation with The Vertical Bar or Pipe Symbol  9. Optional Items  10. Repetition with Star and Plus  11. Use Round Brackets for Grouping 46
  • 47. USE ROUND BRACKETS FOR GROUPING  Grouping the part of the regular expression together for applying a regex operator  Creating a Backreference  reuses part of the regex match  slows down the regex engine  Optimize this regular expression into «Set(?:Value)?  How to Use Backreferences  abc5abc  )[abc])+51$ 1=> a b c - > abbc5c  <div>hello</div>  <([a-z]*)>.*</1> 47
  • 48. REPETITION AND BACKREFERENCES  ([abc]+)» & «([abc])+» to “cab” string  ([abc]+)» : “cab” to be referenced  «([abc])+» : “b” to be referenced 48
  • 49. USE ROUND BRACKETS FOR GROUPING (CONT.)  Reusing the same backreference more than once.  ([a-c])x1x1» „axaxa” „bxbxb” „cxcxc”  Backreferences Cannot be used inside itself.  ([abc]1)  Round brackets Cannot be used inside character classes, as metacharacters.  (a)[1b]  Useful Example: Checking for Doubled Words  «b(w+)s+1b» 49
  • 50. POSIX CLASS Posix ASCII ‫توضیح‬ [:alnum:] [A-Za-z0-9] ‫همه‬‫وعددی‬ ‫حرفی‬ ‫های‬‫اکتر‬‫ر‬‫کا‬ [:alpha:] [A-Za-z] ‫و‬‫بزرگ‬ ‫حروف‬‫کوچک‬ [:blank:] [t] ‫فاصله‬‫تب‬ ‫و‬ [:digit:] [0-9] ‫اعداد‬ [:punct:] [?<=>;:/.,+*()' &%$#”!-] @[]^_`{|}~] ‫عالیم‬‫ی‬ ‫ش‬‫ر‬‫نگا‬ 50
  • 51. ٍEXAMPLE 1  Example 1. Beginning of line ( ^ )  grep "^Nov 10" messages.1  Example 2. End of the line ( $)  grep "terminating.$" messages . Nov 10 01:12:55 gs123 ntpd[2241]: time reset +0.177479 s Nov 10 01:17:17 gs123 ntpd[2241]: synchronized to OCAL(0) Nov 10 01:18:49 gs123 ntpd[2241]: synchronized to 15.1.13.13 Jul 12 17:01:09 cloneme kernel: Kernel log daemon terminating. Oct 28 06:29:54 cloneme kernel: Kernel log daemon terminating 51
  • 52. EXAMPLE 2  Example 3. quantifier (*)(+)(?)  [hc]*at =cchat,hcat,hhhat,at 0 or more  [hc]+at= ccchat, hcat, No at 1 or more  [hc]?at= hat, cat, at 0 or 1  Example 4.Escaping the special character ()  grep "127.0.0.1" /var/log/messages.4 Oct 28 06:31:10 btovm871 ntpd[2241]: Listening on interface lo, 127.0.0.1#123 Enabled 52
  • 53. ٍEXAMPLE 3  Example 5.Excluding specific characters ‫الف‬:  Match text hog  Match text dog ---- > [^b]og  Skip Text bog ‫ب‬:‫غیر‬ ‫ی‬‫اکتر‬‫ر‬‫کا‬a‫یا‬b‫یا‬c [^abc] abccc adb gh 53
  • 54. EXAMPLE 4  Example 6. Composite syntax ‫است‬ ‫یر‬‫ز‬ ‫شرح‬ ‫به‬ ‫الگ‬‫اطالعات‬:  Sun Jun 4 22:08:39 2006 [pid 21611] [dcid] OK LOGIN: Client “192.168.1.1”  ^w+sw+sd+ S+ d+ [pid d+]s [(w+)] OK LOGIN: Client “(d+.d+.d+.d+)”$ 54

Editor's Notes

  • #6: Portable Operating System Interface for uniX
  • #9: When applying «cat» to “He captured a catfish for his cat.”, the engine will try to match the first token in the regex «c» to the first character in the match “H”. This fails. There are no other possible permutations of this regex, because it merely consists of a sequence of literal characters. So the regex engine tries to match the «c» with the “e”. This fails too, as does matching the «c» with the space. Arriving at the 4th character in the match, «c» matches „c”. The engine will then try to match the second token «a» to the 5th character, „a”. This succeeds too. But then, «t» fails to match “p”. At that point, the engine knows the regex cannot be matched starting at the 4th character in the match. So it will continue with the 5th: “a”. Again, «c» fails to match here and the engine carries on. At the 15th character in the match, «c» again matches „c”. The engine then proceeds to attempt to match the remainder of the regex at character 15 and finds that «a» matches „a” and «t» matches „t”. The entire regular expression could be matched starting at character 15. The engine is "eager" to report a match. It will therefore report the first three letters of catfish as a valid match. The engine never proceeds beyond this point to see if there are any “better” matches
  • #11: If you want to use any of these characters as a literal in a regex, you need to escape them with a backslash. If you want to match „1+1=2”, the correct regex is «1+1=2». Otherwise, the plus sign will have a special meaning. Note that «1+1=2», with the backslash omitted, is a valid regex. So you will not get an error message. But it will not match “1+1=2”. It would match „111=2” in “123+111=234”, due to the special meaning of the plus character. If you forget to escape a special character where its use is not allowed, such as in «+1», then you will get an error message.
  • #18: on POSIX bracket expressions for more information.
  • #24: ایزارهای قدیمی خط به خط یک فایل را میخواندند و ریجگس را به آن اعمال میکردند لذا خط جدید متج نمیشد ولی بعدا ابزارهای مدرن کل فایل
  • #26: In the date-matching example, we improved our regex by replacing the dot with a character class. Here, we will do the same. Our original definition of a double-quoted string was faulty. We do not want any number of any character between the quotes. We want any number of characters that are not double quotes or newlines between the quotes. So the proper regex is «"[^" ]*"».
  • #28: Thus far, I have explained literal characters and character classes. In both cases, putting one in a regex will cause the regex engine to try to match a single character. Anchors are a different breed. They do not match any character at all. Instead, they match a position before, after or between characters. They can be used to “anchor” the regex match at a certain position. The caret «^» matches the position before the first character in the string. Applying «^a» to “abc” matches „a”. «^b» will not match “abc” at all, because the «b» cannot be matched right after the start of the string, matched by «^». See below for the inside view of the regex engine. Similarly, «$» matches right after the last character in the string. «c$» matches „c” in “abc”, while «a$» does not match at all.