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\begin{document}
\begin{center}
\textbf{{\LARGE{}Perl and Regular Expressions}}
\end{center}

\begin{center}
{\large{}Regular Expressions are available as part of the programming languages Java, JScript, Visual Basic and VBScript, JavaScript, C, C++, C\#, elisp, Perl, Python, Ruby, PHP, sed, awk, and in many applications, such as editors, grep, egrep.}
\end{center}

\begin{center}
{\large{}Regular Expressions help you master your data.}
\end{center}

\begin{flushleft}
\textbf{{\LARGE{}What is a Regular Expression?}}
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\begin{flushleft}
{\Large{}Powerful.}
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\begin{flushleft}
{\Large{}Low level description:}
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\begin{flushleft}
{\large{}Describes some text}
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\begin{flushleft}
{\large{}Can use to:}
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\begin{flushleft}
Verify a user's input
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\begin{flushleft}
Sift through large amounts of data
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\begin{flushleft}
{\Large{}High level description:}
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\begin{flushleft}
{\large{}Allow you to master your data}
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\begin{flushleft}
\textbf{{\LARGE{}Regular Expressions as a language}}
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\begin{flushleft}
{\large{}Can consider regular expressions as a language}
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\begin{flushleft}
{\large{}Made of two types of characters:}
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\begin{flushleft}
\textit{Literal} characters
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\begin{flushleft}
{\footnotesize{}Normal text characters}
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\begin{flushleft}
{\footnotesize{}Like words of the program}
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\begin{flushleft}
\textit{Metacharacters}
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\begin{flushleft}
{\footnotesize{}The special characters }\textbf{{\footnotesize{}+ ? . * \^{} \$ ( ) [ \{ | \ensuremath{\backslash}}}
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\begin{flushleft}
{\footnotesize{}Act as the grammar that combines with the words according to a set of rules to create and expression that communicates an idea}
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\begin{flushleft}

\end{flushleft}

\begin{center}
\textbf{{\LARGE{}How to use a Regular Expression}}
\end{center}

\begin{center}
{\large{}How to make a regular expression as part of your program}
\end{center}

\begin{flushleft}
\textbf{{\LARGE{}What do they look like?}}
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\begin{flushleft}
{\Large{}In Perl, a regular expression begins and ends with }\textbf{{\Large{}/}}{\Large{}, like this: }\textbf{{\Large{}/abc/}}
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\begin{flushleft}
\textbf{{\Large{}/abc/}}{\Large{} }{\Large{}matches the string "abc"}
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\begin{flushleft}
{\large{}Are these literal characters or metacharacters?}
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\begin{flushleft}
{\Large{}Returns true if matches, so often use as condition in an if statement}
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\begin{flushleft}
\textbf{{\LARGE{}Example: searching for "Course:"}}
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\begin{flushleft}
{\large{}Problem: want to print all lines in all input files that contain the string "Course:"}
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\begin{flushleft}
\textbf{{\footnotesize{}while ( <> ) \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}    my \$line = \$\_;}}
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\begin{flushleft}
\textbf{{\footnotesize{}    if ( \$line =\~{} /Course:/ ) \{}}
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\begin{flushleft}
\textbf{{\footnotesize{}        print \$line;}}
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\begin{flushleft}
\textbf{{\footnotesize{}    \}}}
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\begin{flushleft}
\textbf{{\footnotesize{}\}}}
\end{flushleft}

\begin{flushleft}
{\large{}Or more concisely:}
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\begin{flushleft}
\textbf{{\footnotesize{}while ( <> ) \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}    print if \$\_ =\~{} /Course:/;}}
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\begin{flushleft}
\textbf{{\footnotesize{}\}}}
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\begin{flushleft}
\textbf{{\LARGE{}The "match operator" }}\textbf{{\LARGE{}=\~{}}}
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\begin{flushleft}
{\Large{}If just use /Course:/, this returns true if \$\_ contains the string Course:}
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\begin{flushleft}
{\Large{}If want to test another string variable }\textbf{{\Large{}\$var}}{\Large{}  to see if it contains the regular expression, use}
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\begin{flushleft}
\textbf{{\Large{}\$var =\~{} /regular expression/}}
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\begin{flushleft}
{\Large{}Under what condition is this true?}
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\begin{flushleft}
\textbf{{\LARGE{}The "match operator" }}\textbf{{\LARGE{}=\~{}	   }}\textbf{{\LARGE{}2}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# sets the string to be searched:}}
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\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = "perl for Win32";}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# is 'perl' inside \$\_? }}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( \$\_ =\~{} /perl/ ) \{ print "Found perl\ensuremath{\backslash}n" \};}}
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\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# Same as the regex above.}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# Don't need the =\~{} as we are testing \$\_:}}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( /perl/ )     \{ print "Found perl\ensuremath{\backslash}n" \};}}
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\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}/i}}\textbf{{\LARGE{} Matching without case sensitivity}}
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\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = "perl for Win32";}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# this will fail because the case doesn't match:}}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( /PeRl/ )     \{ print "Found PeRl\ensuremath{\backslash}n" \};}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# this will match, because there is an 'er' in 'perl':}}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( /er/ )       \{ print "Found er\ensuremath{\backslash}n" \};}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# this will match, because there is an 'n3' in 'Win32':}}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( /n3/ )       \{ print "Found n3\ensuremath{\backslash}n" \};}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# this will fail because the case doesn't match:}}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( /win32/ )    \{ print "Found win32\ensuremath{\backslash}n" \};}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# This matches because the /i at the end means}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# "match without case sensitivity":}}
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\begin{flushleft}
\textbf{{\footnotesize{}if ( /win32/i )   \{ print "Found win32 (i)\ensuremath{\backslash}n" \};}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Using }}\textbf{{\LARGE{}!\~{}}}\textbf{{\LARGE{} instead of }}\textbf{{\LARGE{}=\~{}}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# Looking for a space:}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "Found!\ensuremath{\backslash}n"  if      / /;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# both these are the same, but reversing the logic with}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# unless and !\~{}}}
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\begin{flushleft}
\textbf{{\footnotesize{}print "Found!!\ensuremath{\backslash}n" unless \$\_ !\~{} / /;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "Found!!\ensuremath{\backslash}n" unless    ! / /;}}
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\begin{flushleft}

\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Embedding variables in regexps}}
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\begin{flushleft}
\textbf{{\footnotesize{}\# Create two variables containing regular expressions}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# to search for:}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}my \$find = 32;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}my \$find2 = " for ";}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}if ( /\$find/ )  \{ print "Found '\$find'\ensuremath{\backslash}n" \};}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}if ( /\$find2/ ) \{ print "Found '\$find2'\ensuremath{\backslash}n" \};}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# different way to do the above:}}
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\begin{flushleft}
\textbf{{\footnotesize{}print "Found \$find2\ensuremath{\backslash}n" if /\$find2/; }}
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\begin{flushleft}

\end{flushleft}

\begin{center}
\textbf{{\LARGE{}The Metacharacters}}
\end{center}

\begin{center}
{\large{}The funny characters}
\end{center}

\begin{center}
{\large{}What they do}
\end{center}

\begin{center}
{\large{}How to use them}
\end{center}

\begin{flushleft}
\textbf{{\LARGE{}Character Classes		}}\textbf{{\LARGE{}[...]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}my @names = ( "Nick", "Albert", "Alex", "Pick" );}}
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\begin{flushleft}
\textbf{{\footnotesize{}foreach my \$name ( @names ) \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}    if ( \$name =\~{} /[NP]ick/ ) \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}        print "\$name: Out for a Pick Nick\ensuremath{\backslash}n";}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}    else \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}        print "\$name is not Pick or Nick\ensuremath{\backslash}n";}}
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\begin{flushleft}
\textbf{{\footnotesize{}    \}}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\}}}
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\begin{flushleft}
Square brackets \textit{match a single character}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Examples of use of }}\textbf{{\LARGE{}[...]}}
\end{flushleft}

\begin{flushleft}
{\Large{}Match a capital letter: }\textbf{{\Large{}[ABCDEFGHIJKLMNOPQRSTUVWXYZ]}}
\end{flushleft}

\begin{flushleft}
{\Large{}Same thing:}\textbf{{\Large{} [A-Z]}}
\end{flushleft}

\begin{flushleft}
{\Large{}Match a vowel: }\textbf{{\Large{}[aeiou]}}
\end{flushleft}

\begin{flushleft}
{\Large{}Match a letter or digit: }\textbf{{\Large{}[A-Za-z0-9]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Negated character class: }}\textbf{{\LARGE{}[\^{}...]}}
\end{flushleft}

\begin{flushleft}
{\Large{}Match any single character that is }\textit{{\Large{}not}}{\Large{} a letter: }\textbf{{\Large{}[\^{}A-Za-z]}}
\end{flushleft}

\begin{flushleft}
{\Large{}Match any character that is not a space or a tab: }\textbf{{\Large{}[\^{} \ensuremath{\backslash}t]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Example using }}\textbf{{\LARGE{}[\^{}...]}}
\end{flushleft}

\begin{flushleft}
This simple program prints only lines that contain characters that are not a space:
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}while ( <> )}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}\{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}    print \$\_ if /[\^{} ]/;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}\}}}
\end{flushleft}

\begin{flushleft}
This prints lines that \textit{start with} a character that is not a space:
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}while ( <> )}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}\{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}    print \$\_ if /\^{}[\^{} ]/;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}\}}}
\end{flushleft}

\begin{flushleft}
Notice that \textbf{\^{}} has two meanings: one inside \textbf{[...]}, the other outside.
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Shorthand for Common Character Classes}}
\end{flushleft}

\begin{flushleft}
{\large{}Since matching a digit is very common, Perl provides }\textbf{{\large{}\ensuremath{\backslash}d}}{\large{} as a short way of writing \\}\textbf{{\large{}[0-9]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\large{}\ensuremath{\backslash}D}}{\large{} matches a non-digit: }\textbf{{\large{}[\^{}0-9]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\large{}\ensuremath{\backslash}s}}{\large{} matches any whitespace character; shorthand for }\textbf{{\large{}[ \ensuremath{\backslash}t\ensuremath{\backslash}n\ensuremath{\backslash}r\ensuremath{\backslash}f]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\large{}\ensuremath{\backslash}S}}{\large{} non-whitespace, }\textbf{{\large{}[\^{} \ensuremath{\backslash}t\ensuremath{\backslash}n\ensuremath{\backslash}r\ensuremath{\backslash}f]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\large{}\ensuremath{\backslash}w}}{\large{} word character, }\textbf{{\large{}[a-zA-Z0-9\_]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\large{}\ensuremath{\backslash}W}}{\large{} non-word character, }\textbf{{\large{}[\^{} a-zA-Z0-9\_]}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Matching any character}}
\end{flushleft}

\begin{flushleft}
{\Large{}The dot matches any character except a newline}
\end{flushleft}

\begin{flushleft}
{\Large{}This matches any line with at least 5 characters:}
\end{flushleft}

\begin{flushleft}
\textbf{{\Large{}print if /...../;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Matching the beginning or end}}
\end{flushleft}

\begin{flushleft}
{\Large{}to match a line that contains exactly five characters:}
\end{flushleft}

\begin{flushleft}
\textbf{{\Large{}print if /\^{}.....\$/;}}
\end{flushleft}

\begin{flushleft}
{\Large{}the \^{} matches the beginning of the line.}
\end{flushleft}

\begin{flushleft}
{\Large{}the \$ matches at the end of the line}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Matching Repetitions: }}\textbf{{\LARGE{}* + ? \{n,m\}}}
\end{flushleft}

\begin{flushleft}
{\large{}To match zero or more:}
\end{flushleft}

\begin{flushleft}
{\large{}/a*/ will match zero or more letter a, so matches "", "a", "aaaa", "qwereqwqwer", or the nothing in front of }\textit{{\large{}anything}}{\large{}!}
\end{flushleft}

\begin{flushleft}
{\large{}to match at least one:}
\end{flushleft}

\begin{flushleft}
{\large{}/a+/ matches at least one "a"}
\end{flushleft}

\begin{flushleft}
{\large{}/a?/ matches zero or one "a"}
\end{flushleft}

\begin{flushleft}
{\large{}/a\{3,5\}/ matches between 3 and 5 "a"s.}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Example using .*}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = 'Nick Urbanik <nicku@nicku.org>';}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "found something in <>\ensuremath{\backslash}n" if /<.*>/;}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# Find everything between quotes:}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = 'He said, "Hi there!", and then "What\ensuremath{\backslash}'s up?"';}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "quoted!\ensuremath{\backslash}n" if /"[\^{}"]*"/;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "too much!\ensuremath{\backslash}n" if /".*"/;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Capturing the Match with }}\textbf{{\LARGE{}(...)}}
\end{flushleft}

\begin{flushleft}
{\Large{}Often want to scan large amounts of data, extracting important items}
\end{flushleft}

\begin{flushleft}
{\Large{}Use parentheses and regular expressions}
\end{flushleft}

\begin{flushleft}
{\Large{}Silly example of capturing an email address:}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = 'Nick Urbanik <nicku@nicku.org>';}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "found \$1 in <>\ensuremath{\backslash}n" if /<(.*)>/;}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Capturing the match: greediness}}
\end{flushleft}

\begin{flushleft}
{\large{}Look at this example:}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = 'He said, "Hi there!", and then "What\ensuremath{\backslash}'s up?"';}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "\$1\ensuremath{\backslash}n" if /"([\^{}"]*)"/;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "\$1\ensuremath{\backslash}n" if /"(.*)"/;}}
\end{flushleft}

\begin{flushleft}
{\large{}What will each print?}
\end{flushleft}

\begin{flushleft}
{\large{}The first one works; the second one prints:\\}\textbf{{\footnotesize{}Hi there!", and then "What's up?}}
\end{flushleft}

\begin{flushleft}
{\large{}Why?}
\end{flushleft}

\begin{flushleft}
{\large{}Because *, ?, +, \{m,n\} are }\textit{{\large{}greedy}}{\large{}!}
\end{flushleft}

\begin{flushleft}
{\large{}They match as much as they possibly can!}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Being Stingy (not Greedy): }}\textbf{{\LARGE{}?}}
\end{flushleft}

\begin{flushleft}
{\Large{}Usually greedy matching is what we want, but not always}
\end{flushleft}

\begin{flushleft}
{\Large{}How can we match as little as possible?}
\end{flushleft}

\begin{flushleft}
{\Large{}Put a ? after the quantifier:}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}*?		 }}{\footnotesize{}Match 0 or more times}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}+?		 }}{\footnotesize{}Match 1 or more times}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}?? 	 }}{\footnotesize{}Match 0 or 1 time}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\{n,\}?	 }}{\footnotesize{}Match at least n times}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\{n,m\}? }}{\footnotesize{}Match at least n, but no more than m times}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Being Less Greedy: Example}}
\end{flushleft}

\begin{flushleft}
{\Large{}We can solve the problem we saw earlier using non-greedy matching:}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\$\_ = 'He said, "Hi there!", and then "What\ensuremath{\backslash}'s up?"';}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "\$1\ensuremath{\backslash}n" if /"([\^{}"]*)"/;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}print "\$1\ensuremath{\backslash}n" if /"(.*?)"/;}}
\end{flushleft}

\begin{flushleft}
{\Large{}These both work, and match only\\}\textbf{{\footnotesize{}Hi there!}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Sifting through large amounts of data}}
\end{flushleft}

\begin{flushleft}
{\Large{}Imagine you need to create computing accounts for thousands of students}
\end{flushleft}

\begin{flushleft}
{\Large{}As input, you have data of the form:}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}Some heading on the top of each page}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}More headings with other content, including blank lines}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}A tab character separates the columns}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}123456789	H123456(1)}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}234567890	I234567(2)}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}345678901	J345678(3)}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}...		...}}
\end{flushleft}

\begin{flushleft}
\textbf{{\scriptsize{}987654321	A123456(1)}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Capturing the Match:	   }}\textbf{{\LARGE{}(...)}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# useradd() is a function defined elsewhere}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# that creates a computer account with}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# username as first parameter, password as}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\# the second parameter}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}while ( <> ) \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}    if ( /\^{}(\ensuremath{\backslash}d\{9\})\ensuremath{\backslash}t([A-Z]\ensuremath{\backslash}d\{6\}\ensuremath{\backslash}([\ensuremath{\backslash}dA]\ensuremath{\backslash}))/ ) \{}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}        my \$student\_id = \$1;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}        my \$hk\_id = \$2;}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}        useradd( \$student\_id, \$hk\_id );}}
\end{flushleft}

\begin{flushleft}
\textbf{{\footnotesize{}\}}}
\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}The Substitution Operator }}\textbf{{\LARGE{}s///}}
\end{flushleft}

\begin{flushleft}
{\large{}Sometimes want to replace one string with another (editing)}
\end{flushleft}

\begin{flushleft}
{\large{}Example: want to replace Nicholas with Nick on input files:}
\end{flushleft}

\begin{flushleft}
\textbf{while ( <> )}
\end{flushleft}

\begin{flushleft}
\textbf{\{}
\end{flushleft}

\begin{flushleft}
\textbf{    \$\_ =\~{} s/Nicholas/Nick/;}
\end{flushleft}

\begin{flushleft}
\textbf{    print \$\_;}
\end{flushleft}

\begin{flushleft}
\textbf{\}}
\end{flushleft}

\begin{flushleft}

\end{flushleft}

\begin{flushleft}
\textbf{{\LARGE{}Avoiding leaning toothpicks: }}\textbf{{\LARGE{}/\ensuremath{\backslash}/\ensuremath{\backslash}/}}
\end{flushleft}

\begin{flushleft}
{\footnotesize{}Want to change a filename, edit the directory in the path from, say /usr/local/bin/filename to /usr/bin/filename}
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{\footnotesize{}Could do like this:}
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\textbf{{\footnotesize{}s/\ensuremath{\backslash}/usr\ensuremath{\backslash}/local\ensuremath{\backslash}/bin\ensuremath{\backslash}//\ensuremath{\backslash}/usr/\ensuremath{\backslash}bin\ensuremath{\backslash}//;}}
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{\footnotesize{}but this makes me dizzy!}
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{\footnotesize{}We can do this instead:}
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\textbf{{\footnotesize{}s!\ensuremath{\backslash}/usr/local/bin/!/usr/bin/!;}}
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{\footnotesize{}Can use any character instead of }\textbf{{\footnotesize{}/}}{\footnotesize{} in }\textbf{{\footnotesize{}s///}}
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{\footnotesize{}For }\textit{{\footnotesize{}matches}}{\footnotesize{}, can put }\textbf{{\footnotesize{}m//}}{\footnotesize{}, and use any char instead of /}
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{\footnotesize{}Can also use parentheses or braces:}
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\textbf{{\footnotesize{}s\{...\}\{...\}}}{\footnotesize{} or }\textbf{{\footnotesize{}m\{...\}}}
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\textbf{{\LARGE{}Substitution and the }}\textbf{{\LARGE{}/g}}\textbf{{\LARGE{} modifier}}
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{\large{}If an input line contains:}
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\textbf{{\footnotesize{}Nicholas Urbanik read "Nicholas Nickleby"}}
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{\large{}then the output is:}
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\textbf{{\footnotesize{}Nick Urbanik read "Nicholas Nickleby"}}
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{\large{}How change all the Nicholas in one line?}
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{\large{}Use the }\textbf{{\large{}/g}}{\large{}  (global) modifier:}
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\textbf{{\footnotesize{}while ( <> )}}
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\textbf{{\footnotesize{}\{}}
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\textbf{{\footnotesize{}    \$\_ =\~{} s/Nicholas/Nick/g;}}
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\textbf{{\footnotesize{}    print \$\_;}}
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\textbf{{\footnotesize{}\}}}
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\textbf{{\LARGE{}Making regular expressions readable: /x modifier}}
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{\Large{}Sometimes regular expressions can get long, and need comments inside so others (or you later!) understand}
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{\Large{}Use /x at the end of s///x or m//x}
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{\Large{}Allows white space, newlines, comments}
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\end{document}