20. Regular expression special variable
$1, $2, $3 |
hold the backreferences |
$+ |
holds the last (highest-numbered) backreference |
$& |
(dollar ampersand) holds the entire regex match |
$’ |
(dollar followed by an apostrophe or single quote) holds the part of the string after (to the right of) the regex matc |
$` |
(dollar backtick) holds the part of the string before (to the left of) the regex match |
Using these variables is not recommended in perl scripts when performance matters, as it causes Perl to slow down all regex matches in your entire perl script. |
All these variables are read-only, and persist until the next regex match is attempted. |
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$string = "This is the geek stuff article for perl learner"; |
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$string =~ /the (g.) stuff(.) /; |
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print "Matched String=>$&\nBefore Match=>$`\nAfter Match=>$'\nLast Paren=>$+\nFirst Paren=>$1\n"; |
Debugging regexp
use re 'taint';
# Contents of $match are tainted if $dirty was also tainted.
($match) = ($dirty =~ /^(.*)$/s);
# Allow code interpolation:
use re 'eval';
$pat = '(?{ $var = 1 })'; # embedded code execution
/alpha${pat}omega/; # won't fail unless under -T
# and $pat is tainted
use re 'debug'; # like "perl -Dr"
/^(.*)$/s; # output debugging info during
# compile time and run time
use re 'debugcolor'; # same as 'debug',
# but with colored output |
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6 Regular Expressions
($var =~ /re/), ($var !~ /re/) |
matches / does not match |
m/pattern/igmsoxc |
matching pattern |
qr/pattern/imsox |
store regex in variable |
s/pattern/replacement/igmsoxe |
search and replace |
Modifiers: |
i case-insensitive |
o compile once |
g global |
x extended |
s as single line (. matches \n) |
e evaluate replacement |
Syntax: |
\ |
escape |
. |
any single char |
^ |
start of line |
$ |
end of line |
, ? |
0 or more times (greedy / nongreedy) |
+, +? |
1 or more times (greedy / nongreedy) |
?, ?? |
0 or 1 times (greedy / nongreedy) |
\b, \B |
word boundary ( \w - \W) / match except at w.b. |
\A |
string start (with /m) |
\Z |
string end (before \n) |
\z |
absolute string end |
\G |
continue from previous m//g |
[...] |
character set |
(...) |
group, capture to $1, $2 |
(?:...) |
group without capturing |
{n,m} , {n,m}? |
at least n times, at most m times |
{n,} , {n,}? |
at least n times |
{n} , {n}? |
exactly n times |
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or |
\1, \2 |
text from nth group ($1, ...) |
Escape Sequences: |
\a alarm (beep) |
\e escape |
\f formfeed |
\n newline |
\r carriage return |
\t tab |
\cx control-x |
\l lowercase next char |
\L lowercase until \E |
\U uppercase until \E |
\Q diable metachars until \E |
\E end case modifications |
Character Classes: |
[amy] |
'a', 'm', or 'y' |
[f-j.-] |
range f-j, dot, and dash |
[^f-j] |
everything except range f-j |
\d, \D |
digit [0-9] / non-digit |
\w, \W |
word char [a-zA-Z0-9_] / non-word char |
\s, \S |
whitepace [ \t\n\r\f] / non-space |
\C |
match a byte |
\pP, \PP |
match p-named unicode / non-p-named-unicode |
\p{...}, \P{...} |
match long-named unicode / non-named-unicode |
\X |
match extended unicode |
Posix: |
[:alnum:] |
alphanumeric |
[:alpha:] |
alphabetic |
[:ascii:] |
any ASCII char |
[:blank:] |
whitespace [ \t] |
[:cntrl:] |
control characters |
[:digit:] |
digits |
[:graph:] |
alphanum + punctuation |
[:lower:] |
lowercase chars |
[:print:] |
alphanum, punct, space |
[:punct:] |
punctuation |
[:space:] |
whitespace [\s\ck] |
[:upper:] |
uppercase chars |
[:word:] |
alphanum + '_' |
[:xdigit:] |
hex digit |
[:^digit:] |
non-digit |
Extended Constructs |
(?#text) |
comment |
(?imxs-imsx:...) |
enable or disable option |
(?=...), (?!...) |
positive / negative look-ahead |
(?<=..), (?<!..) |
positive / negative look-behind |
(?>...) |
prohibit backtracking |
(?{ code }) |
embedded code |
(??{ code }) |
dynamic regex |
(?(cond)yes|no) |
condition corresponding to captured parentheses |
(?(cond)yes) |
condition corresponding to look-around |
Variables |
$& |
entire matched string |
$` |
everything prior to matched string |
$' |
everything after matched string |
$1, $2 ... |
n-th captured expression |
$+ |
last parenthesis pattern match |
$^N |
most recently closed capt. |
$^R |
result of last (?{...}) |
@-, @+ |
offsets of starts / ends of groups |
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REGEX METACHARS
^ |
string begin |
$ |
str. end (before \n) |
+ |
one or more |
* |
zero or more |
? |
zero or one |
{3,7} |
repeat in range |
() |
capture |
(?:) |
no capture |
[] |
character class |
| |
alternation |
\b |
word boundary |
\z |
string end |
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REGEX MODIFIERS
/i |
case insens. |
/m |
line based ^$ |
/s |
. includes \n |
/x |
ign. wh.space |
/g |
global |
\Q |
quote (disable) pattern metacharacters till \E |
\E |
end either case modification or quoted section, think vi |
REGEX CHARCLASSES
. |
[^\n] |
\s |
[\x20\f\t\r\n] |
\w |
[A-Za-z0-9_] |
\d |
[0-9] |
\S, \W and \D |
negate |
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