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Regular Expressions (view source)

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Most Regular Expression (Regex) implementations are based on or bare similarities to the [[Acronyms#P|PCRE]] library.  The purpose of Regular Expressions are normally to replace or extract bits of data from a larger string.  The syntax of its usage will vary between different languages (eg the functions or constructs that let you use Regex in Perl, PHP, PowerShell, etc), but the syntax of the actual Regex will ''generally'' be consistent.  The secret to creating good Regex is good and thorough testing for things that ''shouldn't'' match as much as ''should'' match.


== Random ==
The following provides a run through of the general syntax (its not exhaustive, its a bit of a work in progress, but contains most of the common stuff).
* '''Filename'''


See the [[Regex Examples]] page for some real world examples.


== Useful/Standard RegEx ==
== Metacharacters ==
{|cellpadding="2" cellspacing="0" border="1"  
Metacharacters define how an item should match (or not)
|- style="background-color:#bbddff;"
{|class="vwikitable"
! Matches                  !! Expression
|-  
! Character !! Meaning !! Example
|-
|-
| '''IP Address'''
| <code>\</code> || Escape
| <code><nowiki> ^\b((25[0-5]|2[0-4]\d|[01]\d\d|\d?\d)\.){3}(25[0-5]|2[0-4]\d|[01]\d\d|\d?\d)\b </nowiki></code>
|-
|-
| '''Hostname''' (no domain)
| <code>^</code> || Match the beginning of the line || <code>^dog</code> matches <code>dog</code> but not <code>big dog</code>
| <code><nowiki> \A(\w|-)+ </nowiki></code>
|-
|-
| '''Email address'''
| <code>$</code> || Match the end of the line || <code>dog$</code> matches <code>dog</code> but not <code>dog run</code>
| <code><nowiki> \b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b </nowiki></code>
|-
| <code>.</code> || Match any character || <code>do.</code> matches <code>do</code>, <code>dog</code> but not <code>dogs</code>
|-
| <code>(...)</code> || Grouping, match entire set <code>...</code> ||
|-
| <code>[...]</code> || Bracketed [[#Character_Classes|Character class]], any character contained in set <code>...</code> || <code>[fld]og*</code> matches <code>fog</code>, <code>log</code>, or <code>dog</code> only
|-
| <code><nowiki>|</nowiki></code> || Or, alternation || <code>d<nowiki>|</nowiki>fog</code> matches <code>dog</code> or <code>fog</code>
|-
| <code>?=</code>      || Lookahead                    || <code>dog(?=,)</code> matches the <code>dog</code> in <code>dog,</code> but won't find a match in <code>dog</code>
|-
| <code>?<=</code>      || Look behind                  || <code>(?<=big )dog</code> matches the <code>dog</code> in <code>big dog</code> but won't find a match in <code>dog</code>
|}
|}


== Examples ==
== Quantifiers ==
=== Logfile Name ===
Quantifiers define how many times the preceding item should be matched.
''' <code> \d{4}-[A-Za-z]{3}-Week\d{1}.log </code> '''
{|class="vwikitable"
|-
! Character !! Meaning !! Example
|-
| <code>*</code> || Match 0 or more times || <code>dog*</code> matches <code>do</code>, <code>dog</code>, <code>dogg</code>, or <code>doggg</code>, and so on
|-
| <code>+</code> || Match 1 or more times || <code>dog+</code> matches <code>dog</code>, <code>dogg</code>, or <code>doggg</code>, and so on; but not <code>do</code>
|-
| <code>?</code> || Match 0 or 1 times only || <code>dog?</code> matches <code>do</code> or <code>dog</code> only
|-
| <code>{n}</code> || Match exactly <code>n</code> times || <code>dog{2}</code> matches <code>dogg</code> only
|-
| <code>{n,}</code> || Match <code>n</code> or more times || <code>dog{2,}</code> matches <code>dogg</code>, or <code>doggg</code>, and so on
|-
| <code>{n,m}</code> || Match <code>n</code> up to <code>m</code> times || <code>dog{2,4}</code> matches <code>dogg</code>, <code>doggg</code>, or <code>dogggg</code> only
|}


Example matches...
== Character Classes ==
* <code> 2010-Feb-Week4.log </code>
Character Classes define a set of characters that should be matched
* <code> 2009-Dec-Week2.log </code>
{|class="vwikitable"
* <code> 1234-aBc-Week0.log </code>
|-
! Sequence !! Matches !! Example
|-
| <code>[...]</code> || Any character contained in set <code>...</code> || <code>[fld]og*</code> matches <code>fog</code>, <code>log</code>, or <code>dog</code> only
|-
| <code>[:...:]</code> || Any character defined by [[#POSIX Classes|POSIX class]] <code>...</code> ||
|-
| <code>\w</code> || Any word character (alphanumeric and underscore) ||
|-
| <code>\W</code> || Any non-word character ||
|-
| <code>\s</code> || Any white-space character ||
|-
| <code>\S</code> || Any non-whitespace character ||
|-
| <code>\d</code> || Any decimal digit character ||
|-
| <code>\D</code> || Any non-decimal digit character ||
|}


=== Between Parentheses ===
=== POSIX Classes ===
''' <code> (?<=\[)(.*?)(?=\]) </code> '''
[[Acronyms#P|POSIX]] classes are generally not supported by Microsoft implementations.
 
{|class="vwikitable"
Matches everything between <code> [ </code> and <code> ] </code>, so for example...
|-
* <code> VMFS_SCSI_DS_01 </code> is matched from <code> [VMFS_SCSI_DS_01] My_VM/MyVM.vmdk
! Class !! Matches !! Equivalent to
|-
| <code>alpha</code> || Any alphabetical character || <code>[A-Za-z]</code>
|-
| <code>alnum</code> || Any alphanumeric character || <code>[A-Za-z0-9]</code>
|-
| <code>ascii</code> || Any [[Acronyms#A|ASCII]] character ||
|-
| <code>blank</code> || A space or tab ||
|-
| <code>cntrl</code> || Control characters ||
|-
| <code>digit</code> || Any decimal digit || <code>[0-9]</code> or <code>\d</code>
|-
| <code>graph</code> || Any graphical/visible character ||
|-
| <code>lower</code> || Any lower-case alphabetical character || <code>[a-z]</code>
|-
| <code>print</code> || Any printable character, including a space ||
|-
| <code>punct</code> || Any punctuation character ||
|-
| <code>space</code> || Any white-space character ||
|-
| <code>upper</code> || Any upper-case character || <code>[A-Z]</code>
|-
| <code>word</code> || Any word character || <code>[A-Za-z0-9_]<\code> or <code>\w</code>
|-
| <code>xdigit</code> || Any hexadecimal digit || <code>[0-9a-fA-F]</code>
|}


Its essentially done via three chunks of the regex...
[[Category:Regex]]
# <code> (?<=\[) </code>
#* Requires that <code> [ </code> immediately proceeds the match
# <code> (.*?) </code>
#* Matches everything
# <code> (?=\]) </code>
#* Requires that <code> [ </code> immediately follows the match
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