Presentation is loading. Please wait.

Presentation is loading. Please wait.

LING/C SC/PSYC 438/538 Computational Linguistics Sandiway Fong Lecture 2: 8/23.

Similar presentations

Presentation on theme: "LING/C SC/PSYC 438/538 Computational Linguistics Sandiway Fong Lecture 2: 8/23."— Presentation transcript:

1 LING/C SC/PSYC 438/538 Computational Linguistics Sandiway Fong Lecture 2: 8/23

2 Administrivia Anyone try installing Perl yet? –Active State Perl –http://www.activestate.com –install the free version –(not the trial versions)

3 Background Reading Textbook –Chapter 2: Regular Expressions and Automata –Section 2.1: Regular Expressions Perl Regular Expressions (RE) –perlrequick - Perl regular expressions quick startperlrequick –perlretut - Perl regular expressions tutorialperlretut

4 Regular Expressions regular expressions are used in string pattern-matching –important tool in automated searching –formally equivalent to finite-state automata (FSA) and regular grammars popular implementations –Unix grep command line program returns lines matching a regular expression standard part of all Unix-based systems –including MacOS X (command-line interface in Terminal) many shareware/freeware implementations available for Windows XP –just Google and see... –grep functionality is built into many programming languages e.g. Perl –wildcard search in Microsoft Word limited version of regular expressions (not full power) with differences in notation

5 Regular Expressions Historical note –grep : name comes from Unix ed command –g/re/p –“search globally for lines matching the regular expression, and print them” –[Source:] –ed is an obscure and difficult-to- use text edit program on Unix systems –doesn’t need a screen display –would work on an ancient teletype wikipedia

6 Regular Expressions Formally –a regular expression (regexp) is formed from: an alphabet (= set of characters) operators –a regexp is shorthand for a set of strings (possibly infinite set) (strings are of finite length) Formally –a set of strings is called a language –a language that can be defined by a regular expression is called a regular language (not all languages are regular)

7 Regular Expressions alphabet –e.g. {a,b,c,...,z} set of lower case English letters Note: case is important operators –asingle symbol a –a n exactly n occurrences of a, n a positive integer –a n a 3  aaa –a * zero or more occurrences of a –a + one or more occurrences of a –concatenation two regexps may be concatenated, the resulting string is also a regexp e.g. abc –disjunction infix operator: | (vertical bar) e.g. a|b –parentheses may be used for disambiguation e.g. gupp(y|ies)

8 Regular Expressions Technically, a + is not necessary –aa* = a + “a concatenated with a* (zero or more occurrences of a)” = “one or more occurrences of a” Disjunction –[set of characters] set of characters enclosed in square brackets means match one of the characters –e.g. [aeiou] matches any of the vowels a, e, i, o or u but not d –dash (-) shorthand for a range –e.g. [a-e] matches a, b, c, d or e

9 Regular Expressions Range defined over a computer character set –typically ASCII –originally a 7 bit character set –2^7 = 128 (0-127) different characters –ASCII = American Standard Code for Information Interchange –e.g. [A-z] [0-9A-Za-z] scii_table/PROGRAMMING_ascii_table.shtml

10 Regular Expressions: Microsoft Word terminology: –wildcard search

11 Regular Expressions: Microsoft Word Note: zero or more times is missing in Microsoft Word

12 Regular Expressions Perl uses the same notation as grep (textbook also uses grep notation) More shorthand –question mark (?) means the previous regexp is optional –e.g. colou?r –matches color or colour –metacharacters or operators like ? have a function –to match a question mark, escape it using a backslash (\) –e.g. why\? –? in Microsoft Word means match any character More shorthand –period (.) stands for any character (except newline) –e.g. e.t matches eat as well as eet –caret sign (^) as the first character of a range of characters [^set of characters] means don’t match any of the characters mentioned (after the caret) –e.g. [^aeiou] –any character except for one of the vowels listed

13 Regular Expressions Text files in Unix consists of sequences of lines separated by a newline character (LF = line feed) Typically, text files are read a line at a time by programs Matching in Perl and grep is line-oriented (can be changed in Perl) Differences in platforms for line breaking: –Unix: LF –Windows (DOS): CR LF –MacOS (X): CR affects binary file transfers

14 Regular Expressions Line-oriented metacharacters: –caret (^) at the beginning of a regexp string matches the “beginning of a line” –e.g. ^The matches lines beginning with the sequence The –Note: the caret is very overloaded... [^ab] a^b –dollar sign ($) at the end of a regexp string matches the “end of the line” –e.g. end\.$ –matches lines ending in the sequence end. –e.g. ^$ matches blank lines only –e.g. ^ $ matches lines contains exactly one space

15 Regular Expressions Word-oriented metacharacters: –a word is any sequence of digits [0-9], underscores (_) and letters [a-zA-Z] –(historical reasons for this) –\b matches a word boundary, e.g. a space or beginning or end of a line or a non-word character –e.g. the –matches the, they, breathe and other –but \bthe will only match the and they –the\b will match the and breathe –\bthe\b will only match the –(\ can also be used to match the beginning and end of a word) –e.g. \b99 –matches 99 but not 299 –also matches $99

16 Regular Expressions Note: –definition of word does not include characters with accent marks extended ASCII character set –8 bit –characters 128-255 –does not include non-Roman characters towards multilingual computing –Unicode –one massive table

17 Regular Expressions Range abbreviations: –\d (digit) = [0-9] –\s (whitespace character) = space (SP), tab (HT), carriage return (CR), newline (LF) or form feed (FF) –\w (word character) = [0-9a-zA-Z_] uppercase versions denote negation –e.g. \W means a non-word character \D means a non-digit Repetition abbreviations: –a? a optional –a* zero or more a’s –a+ one or more a’s –a{n,m} between n and m a’s –a{n,} at least n a’s –a{n} exactly n a’s –e.g. \d{7,} matches numbers with at least 7 digits –e.g. \d{3}-\d{4} –matches 7 digit telephone numbers with a separating dash

18 Perl Run from the command line in Windows –Start > Run... –cmd (brings up command line interpreter) Running a Perl program: –perl -help (gives options) –perl (runs Perl command file ) –perl inputfile.txt (runs Perl command file, inputfile.txt is supplied to ) e.g. reads and processes input file inputfile.txt

19 Perl Example Perl program ( ) to read in a text file and print lines matching a regexp enclosed by /.../ Example input file ( text.txt ) Command perl text.txt open (F,$ARGV[0]) or die "$ARGV[0] not found!\n"; while ( ) { print $_ if (/The/); } This is a test. The cat sat on the mat. These shoes are made for walking. Otherwise, I thought it was cold. 45

20 Perl Program: open (F,$ARGV[0]) or die "$ARGV[0] not found!\n"; while ( ) { print $_ if (/The/); } –while ( ) first evaluates – reads in a line from the file referenced by F and places the line in the program variable $_ –then it executes the program code between the curly braces –then it goes back and reads another line –it does this repeatedly while produces a valid line – if we reach the end of the file, the while loop stops –print $_ if (/.../); is conditional code that means print the contents of variable $_ if the regexp between the /.../ can be found in $_ Program explained: – open the file referenced in $AGRV[0] for input – $AGRV[0] is the first command line argument following the program name – F is the file descriptor associated with the opened file – if there is a problem opening the file, e.g. file doesn’t exist, program execution dies and prints the value of the string enclosed in double quotes "$ARGV[0] not found!\n"

Download ppt "LING/C SC/PSYC 438/538 Computational Linguistics Sandiway Fong Lecture 2: 8/23."

Similar presentations

Ads by Google