CS1655 Recitation 2

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CS1655 Recitation 2

• Andreea Munteanu
• Andreea_at_cs.pitt.edu

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Overview

• Perl regular expressions.
• Wget.
• Examples.

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Perl regular expression (1)

• Extremely Powerful Text Processing.
• Test if a string or its substring matches with
  some pattern.
• Replace or substitute some string pattern in a
  text string.
• Extract substring from a string based on certain
  text pattern.

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Perl regular expression (2)

• Comparison with a string
• if (var /BLAH/) Match string.
• if (var /BLAH/) Start of String.
• if (var /BLAH/) End of String.
• if (var /\w/) Any letters.

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Perl regular expression (3)

• Items that match  a single character in a
  predefined character class
• \w  Match a "word" character.
• (alphanumeric  plus "_").
• \W  Match a non-word character.
• \s  Match a whitespace character.
• \S  Match a non-whitespace character.
• \d  Match a digit character.
• \D  Match a non-digit character.

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Perl regular expression (4)

• Items to match a single character
• . dot Match any one
  characters.
• ... character class Match any
  character listed.
• ... negated character class Match any
  character not listed
• \t tab Match HT or TAB character.
• \n new line Match LF or NL character.
• \r return Match CR character.

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Perl regular expression (5)

• Quantity symbols
• Match 0 or more times.
• Match 1 or more times.
• ? Match 0 or 1 times.
• n Match exactly n times.
• n, Match at least n times.
• n,m Match at least n times, but no more
  than m times.

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Perl regular expression (6)

• Metacharacters
• These need to be escaped to be matched
• \ . ? ( )

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Examples

• Example 1 (Character class)
• if(string /010-9/)
• print string contains digits 00 to 19\n"
• elseprint "string does not contain digits 00 to
  19\n"

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Perl regular expression substitution

• Substitution is specified with s.
• string s/apples/oranges/
• Wherever "apples" is found in string, it will be
  replaced by
• "oranges". Note it replaces only the first
  appearance of the
• searched string.
• To replace all appearances, use a g at the end
• string s/apples/oranges/g
• To ignore case-sensitivity, use i at the end
• string s/apples/oranges/gi

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Perl substring extraction

• Example problem. Given the path
• c/anypath1/Apath2/Bpath3/code01038_18e.jpg
• we want to extract "code01038_18e" and then to
  modify it to "code01038_18a".

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Example solution

• !/usr/bin/perl
• source "c/anypath1/Apath2/Bpath3/code01038_18
  e.jpg"
• print "source\n" Extract everything from the
  last / onward
• ind rindex(source, "/")
• tmp1 substr(source, ind 1, -4)
• Remove the last character and replace with 'a'
• print "tmp1\n"
• tmp2 substr(tmp1, 0, -1) . 'a'
• print "tmp2\n"

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Wget

• A network utility to retrieve files from the Web
  using http and ftp protocols.
• Works non-interactively, in the background, even
  after having logged off.
• Works well with slow or unstable connections by
  continuing to retrieve a document until the
  document is fully downloaded.

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Examples Wget (1)

• Download a url gt just type
• wget http//fly.srk.fer.hr/
• The connection will probably fail before the
  whole file is retrieved gt retry a number of
  times
• wget --tries45 http//fly.srk.fer.hr/jpg/flyweb.
  jpg

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Examples Wget (2)

• Leave Wget to work in the background, and write
  its progress to log file log
• wget -t 45 -o log http//fly.srk.fer.hr/jpg/flywe
  b.jpg
• If you specify a directory gt retrieve the
  directory listing, parse it and convert it to
  html. Try
• wget ftp//ftp.gnu.org/pub/gnu/ links
  index.html
• To unlimit the number of retries -t inf.