Perl Learning notes-Regular expressions

Regular expressions

Simple mode: Matches the contents of the $_, just write the pattern in a pair of slashes (/).

such as: #!/usr/bin/env Perl

Use 5.010;

$_= "Yabba dabba Doo";

if (/abba/) {

Say "it matched!";

}

About meta characters

similar to the Shell :

.==> any character;

*==> repeats 0 times and more than 0 times;

+==> repeat once and more;

?==> repeat 0 or one time;

Pattern grouping

In a regular expression, groups the strings with parentheses () .

The inverse reference is written as a numeric number followed by a slash, such as \1 \2 . The corresponding number represents the capturing group for the corresponding order.

The following examples illustrate:

The reverse reference does not have to be in the back of the corresponding capturing group. The following pattern or 4 consecutive non-newline characters followed by y , and a \1 Reverse Reference indicates that the a 4 -character case.

$_= "Yabba dabba Doo";

If (/y (...) d\1/) {

Print "It matched the same after Y and d!\n";

}

You can also use multiple parentheses to separate groups, each of which can have its own reverse reference.

$_= "Yabba dabba Doo";

If (/y (.) (.) \2\1/) {# match ' Abba '

Print "It matched the same after Y and d!\n";

}

So how do you tell which parentheses are the group? Larry explanation: Just one point at the left parenthesis ( including nested parentheses ) The serial number is OK up. such as:

$_= "Yabba dabba Doo";

if (/y (.) (.) \3\2) d\1/) {

Print "It matched the same after Y and d!\n";

}

Disassemble: ( # The first parenthesis

(.) # a second parenthesis

(.) # A third parenthesis

\3

\2

)

The problems left above:

$_= "AA11BB";

If (/(.) \111/) {

Print "It matched!\n";

}

Originally intended to match Aa11 , now good. Perl interpreted it as matching section 111 brackets, and could not find this parenthesis at all, error.

Workaround:\g{1} can eliminate the two semantics of the inverse reference and the direct volume portion of the pattern.

Use 5.010;

$_= "AA11BB";

If (/(.) \g{1}11/) {

Print "It matched." \ n ";

}

And with \g{n} There is also a benefit is that N can be negative, that is, can be -1-2 and so on. The meaning of the expression is the reciprocal or relative position.

-1 represents The nearest opening parenthesis from \g{-1} ;

-2 represents The nearest second left parenthesis from \g{-2} ;

Use 5.010;

$_= "AA11BB";

if (/(.) (.) \g{-1}11/) {

Print "It matched." \ n ";

}

Choose a match

(| )

such as:/fred (and|or) barney/

Character

[A-za-z]

[^a-za-z]

Shorthand for character sets

A shorthand \dfor a character set that represents any number;

$_= ' The HAL-9000 requires authorization to continue. ';

if (/hal-[\d]+/) {

Say "It matched."

}

The modifier/a , written at the end of the regular expression, is expanded according to the semantics of ASCII ( from Perl 5.14 Modifiers introduced ):

Use 5.014;

$_= ' The HAL-9000 requires authorization to continue. ';

If (/hal-[\d]+/a) {# my old ASCII character explained

Say "It matched."

}

Description: The main reason for introducing/A is that the semantics of \d now is not only the scope of [0-9] , it also represents a number of more special characters.

\s can match the following 5 whitespace characters: page break \f horizontal tab \v vertical tab \h Carriage return \ n whitespace \p

Use 5.014;

if (/\s/a) {# semantics explained by the old ASCII Word

Say "The string matched ASCII whitescape";

}

\ r matches a break, either \ r \ n or \ nthe match.

\w matches " word " characters, so-called words are actually made up of [a-za-z0-9] .

Anti-semantic shorthand

\d said [^\d]

\w said [^\w]

\s said [^\s]

These abbreviations can be used as separate character sets in a pattern, or as part of a character set in square brackets. For example:/[\da-fa-f]/

Using regular expressions for matchingm//to match

The previous use // notation pattern, such as /fred/ , is actually a special case of m// .

Common like QW// , separators are also optional, such as:m{fred} m <fred>

Pattern modifiers

/a means expansion in accordance with The semantics of ASCII

/I indicates case-insensitive matching

Print "would to play a game?";

Chomp ($_=<stdin>);

If (/yes/i) {# case-insensitive matching

Say "I like too.";

}

/s means matching any character;

In many cases (.) There is no way to match a newline character, but if the string contains a newline character and you want to match those line breaks, you can use the / S modifier is complete. (The implementation principle is thatPerl Converts the dot number to a character set [\d\d] to handle, meaning it matches any character)

$_= "I saw barney\n down at the bowling alley\nwith fred\nlast night.\n";

If (/barney.*fred/s) {

Print "that string mentions Fred after barney\n";

}

The problem arises:/s will put the pattern in place . are modified to match any character, so what if we just want to match a few of them with any character? ? can be used \ n

/x indicates the addition of whitespace characters;

Examples are as follows:

#!/usr/bin/env Perl

Use 5.010;

$_= "Fred";

if (/fre d/x) {

Say "it matched."

}

Of course, the above modifiers can be combined:

if (/barney.*fred/is) {# use both /I and/ s

Print "that string mentions Fred after barney!\n";

}

Perl Learning notes-Regular expressions