perlrequick.1

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.\" ========================================================================
.\"
.IX Title "PERLREQUICK 1"
.TH PERLREQUICK 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
perlrequick \- Perl regular expressions quick start
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This page covers the very basics of understanding, creating and
using regular expressions ('regexes') in Perl.
.SH "The Guide"
.IX Header "The Guide"
.Sh "Simple word matching"
.IX Subsection "Simple word matching"
The simplest regex is simply a word, or more generally, a string of
characters.  A regex consisting of a word matches any string that
contains that word:
.PP
.Vb 1
\&    "Hello World" =~ /World/;  # matches
.Ve
.PP
In this statement, \f(CW\*(C`World\*(C'\fR is a regex and the \f(CW\*(C`//\*(C'\fR enclosing
\&\f(CW\*(C`/World/\*(C'\fR tells perl to search a string for a match.  The operator
\&\f(CW\*(C`=~\*(C'\fR associates the string with the regex match and produces a true
value if the regex matched, or false if the regex did not match.  In
our case, \f(CW\*(C`World\*(C'\fR matches the second word in \f(CW"Hello World"\fR, so the
expression is true.  This idea has several variations.
.PP
Expressions like this are useful in conditionals:
.PP
.Vb 1
\&    print "It matches\en" if "Hello World" =~ /World/;
.Ve
.PP
The sense of the match can be reversed by using \f(CW\*(C`!~\*(C'\fR operator:
.PP
.Vb 1
\&    print "It doesn\*(Aqt match\en" if "Hello World" !~ /World/;
.Ve
.PP
The literal string in the regex can be replaced by a variable:
.PP
.Vb 2
\&    $greeting = "World";
\&    print "It matches\en" if "Hello World" =~ /$greeting/;
.Ve
.PP
If you're matching against \f(CW$_\fR, the \f(CW\*(C`$_ =~\*(C'\fR part can be omitted:
.PP
.Vb 2
\&    $_ = "Hello World";
\&    print "It matches\en" if /World/;
.Ve
.PP
Finally, the \f(CW\*(C`//\*(C'\fR default delimiters for a match can be changed to
arbitrary delimiters by putting an \f(CW\*(Aqm\*(Aq\fR out front:
.PP
.Vb 4
\&    "Hello World" =~ m!World!;   # matches, delimited by \*(Aq!\*(Aq
\&    "Hello World" =~ m{World};   # matches, note the matching \*(Aq{}\*(Aq
\&    "/usr/bin/perl" =~ m"/perl"; # matches after \*(Aq/usr/bin\*(Aq,
\&                                 # \*(Aq/\*(Aq becomes an ordinary char
.Ve
.PP
Regexes must match a part of the string \fIexactly\fR in order for the
statement to be true:
.PP
.Vb 3
\&    "Hello World" =~ /world/;  # doesn\*(Aqt match, case sensitive
\&    "Hello World" =~ /o W/;    # matches, \*(Aq \*(Aq is an ordinary char
\&    "Hello World" =~ /World /; # doesn\*(Aqt match, no \*(Aq \*(Aq at end
.Ve
.PP
perl will always match at the earliest possible point in the string:
.PP
.Vb 2
\&    "Hello World" =~ /o/;       # matches \*(Aqo\*(Aq in \*(AqHello\*(Aq
\&    "That hat is red" =~ /hat/; # matches \*(Aqhat\*(Aq in \*(AqThat\*(Aq
.Ve
.PP
Not all characters can be used 'as is' in a match.  Some characters,
called \fBmetacharacters\fR, are reserved for use in regex notation.
The metacharacters are
.PP
.Vb 1
\&    {}[]()^$.|*+?\e
.Ve
.PP
A metacharacter can be matched by putting a backslash before it:
.PP
.Vb 4
\&    "2+2=4" =~ /2+2/;    # doesn\*(Aqt match, + is a metacharacter
\&    "2+2=4" =~ /2\e+2/;   # matches, \e+ is treated like an ordinary +
\&    \*(AqC:\eWIN32\*(Aq =~ /C:\e\eWIN/;                       # matches
\&    "/usr/bin/perl" =~ /\e/usr\e/bin\e/perl/;  # matches
.Ve
.PP
In the last regex, the forward slash \f(CW\*(Aq/\*(Aq\fR is also backslashed,
because it is used to delimit the regex.
.PP
Non-printable \s-1ASCII\s0 characters are represented by \fBescape sequences\fR.
Common examples are \f(CW\*(C`\et\*(C'\fR for a tab, \f(CW\*(C`\en\*(C'\fR for a newline, and \f(CW\*(C`\er\*(C'\fR
for a carriage return.  Arbitrary bytes are represented by octal
escape sequences, e.g., \f(CW\*(C`\e033\*(C'\fR, or hexadecimal escape sequences,
e.g., \f(CW\*(C`\ex1B\*(C'\fR:
.PP
.Vb 2
\&    "1000\et2000" =~ m(0\et2)        # matches
\&    "cat"        =~ /\e143\ex61\ex74/ # matches, but a weird way to spell cat
.Ve
.PP
Regexes are treated mostly as double quoted strings, so variable
substitution works:
.PP
.Vb 3
\&    $foo = \*(Aqhouse\*(Aq;
\&    \*(Aqcathouse\*(Aq =~ /cat$foo/;   # matches
\&    \*(Aqhousecat\*(Aq =~ /${foo}cat/; # matches
.Ve
.PP
With all of the regexes above, if the regex matched anywhere in the
string, it was considered a match.  To specify \fIwhere\fR it should
match, we would use the \fBanchor\fR metacharacters \f(CW\*(C`^\*(C'\fR and \f(CW\*(C`$\*(C'\fR.  The
anchor \f(CW\*(C`^\*(C'\fR means match at the beginning of the string and the anchor
\&\f(CW\*(C`$\*(C'\fR means match at the end of the string, or before a newline at the
end of the string.  Some examples:
.PP
.Vb 5
\&    "housekeeper" =~ /keeper/;         # matches
\&    "housekeeper" =~ /^keeper/;        # doesn\*(Aqt match
\&    "housekeeper" =~ /keeper$/;        # matches
\&    "housekeeper\en" =~ /keeper$/;      # matches
\&    "housekeeper" =~ /^housekeeper$/;  # matches
.Ve
.Sh "Using character classes"
.IX Subsection "Using character classes"
A \fBcharacter class\fR allows a set of possible characters, rather than
just a single character, to match at a particular point in a regex.
Character classes are denoted by brackets \f(CW\*(C`[...]\*(C'\fR, with the set of
characters to be possibly matched inside.  Here are some examples:
.PP
.Vb 3
\&    /cat/;            # matches \*(Aqcat\*(Aq
\&    /[bcr]at/;        # matches \*(Aqbat\*(Aq, \*(Aqcat\*(Aq, or \*(Aqrat\*(Aq
\&    "abc" =~ /[cab]/; # matches \*(Aqa\*(Aq
.Ve
.PP
In the last statement, even though \f(CW\*(Aqc\*(Aq\fR is the first character in
the class, the earliest point at which the regex can match is \f(CW\*(Aqa\*(Aq\fR.
.PP
.Vb 3
\&    /[yY][eE][sS]/; # match \*(Aqyes\*(Aq in a case\-insensitive way
\&                    # \*(Aqyes\*(Aq, \*(AqYes\*(Aq, \*(AqYES\*(Aq, etc.
\&    /yes/i;         # also match \*(Aqyes\*(Aq in a case\-insensitive way
.Ve
.PP
The last example shows a match with an \f(CW\*(Aqi\*(Aq\fR \fBmodifier\fR, which makes
the match case-insensitive.
.PP
Character classes also have ordinary and special characters, but the
sets of ordinary and special characters inside a character class are
different than those outside a character class.  The special
characters for a character class are \f(CW\*(C`\-]\e^$\*(C'\fR and are matched using an
escape:
.PP
.Vb 5
\&   /[\e]c]def/; # matches \*(Aq]def\*(Aq or \*(Aqcdef\*(Aq
\&   $x = \*(Aqbcr\*(Aq;
\&   /[$x]at/;   # matches \*(Aqbat, \*(Aqcat\*(Aq, or \*(Aqrat\*(Aq
\&   /[\e$x]at/;  # matches \*(Aq$at\*(Aq or \*(Aqxat\*(Aq
\&   /[\e\e$x]at/; # matches \*(Aq\eat\*(Aq, \*(Aqbat, \*(Aqcat\*(Aq, or \*(Aqrat\*(Aq
.Ve
.PP
The special character \f(CW\*(Aq\-\*(Aq\fR acts as a range operator within character
classes, so that the unwieldy \f(CW\*(C`[0123456789]\*(C'\fR and \f(CW\*(C`[abc...xyz]\*(C'\fR
become the svelte \f(CW\*(C`[0\-9]\*(C'\fR and \f(CW\*(C`[a\-z]\*(C'\fR:
.PP
.Vb 2
\&    /item[0\-9]/;  # matches \*(Aqitem0\*(Aq or ... or \*(Aqitem9\*(Aq
\&    /[0\-9a\-fA\-F]/;  # matches a hexadecimal digit
.Ve
.PP
If \f(CW\*(Aq\-\*(Aq\fR is the first or last character in a character class, it is
treated as an ordinary character.
.PP
The special character \f(CW\*(C`^\*(C'\fR in the first position of a character class
denotes a \fBnegated character class\fR, which matches any character but
those in the brackets.  Both \f(CW\*(C`[...]\*(C'\fR and \f(CW\*(C`[^...]\*(C'\fR must match a
character, or the match fails.  Then
.PP
.Vb 4
\&    /[^a]at/;  # doesn\*(Aqt match \*(Aqaat\*(Aq or \*(Aqat\*(Aq, but matches
\&               # all other \*(Aqbat\*(Aq, \*(Aqcat, \*(Aq0at\*(Aq, \*(Aq%at\*(Aq, etc.
\&    /[^0\-9]/;  # matches a non\-numeric character
\&    /[a^]at/;  # matches \*(Aqaat\*(Aq or \*(Aq^at\*(Aq; here \*(Aq^\*(Aq is ordinary
.Ve
.PP
Perl has several abbreviations for common character classes:
.IP "\(bu" 4
\&\ed is a digit and represents
.Sp
.Vb 1
\&    [0\-9]
.Ve
.IP "\(bu" 4
\&\es is a whitespace character and represents
.Sp
.Vb 1
\&    [\e \et\er\en\ef]
.Ve
.IP "\(bu" 4
\&\ew is a word character (alphanumeric or _) and represents