regex is fun

Regex is Fun

David ClawsonSplunkYoda

Regular Expressions

• “A regular expression is a pattern which specifies a set of

strings of characters; it is said to match certain strings.”

—Ken Thompson

• QED Text Editor written by Ken in the 1970s

• Invented in the 1940s

• Help celebrate it’s 70th Year

Types of Regular Expressions

How is Regex used in Python?

Python “re”

Python's built-in "re" module provides excellent support for regular expressions, with a

modern and complete regex flavor.

The only significant features missing from Python's regex syntax are atomic grouping,

possessive quantifiers, and Unicode properties.

Using Regular Expressions in Python

The first thing to do is to import the regexp module into your script with “import re”.

Call re.search(regex, subject) to apply a regex pattern to a subject string. • The function returns None if the matching attempt fails, and a Match object otherwise. • The Match object stores details about the part of the string matched by the regular expression

pattern.

Since None evaluates to False, you can easily use re.search() in an if statement.

Do not confuse re.search() with re.match(). • Both functions do exactly the same, with the important distinction that re.search() will attempt

the pattern throughout the string, until it finds a match. re.match() on the other hand, only attempts the pattern at the very start of the string.

To get all matches from a string, call re.findall(regex, subject). This will return an array of all non-overlapping regex matches in the string. "Non-overlapping" means that the string is searched through from left to right, and the next match attempt starts beyond the previous match.

If the regex contains one or more capturing groups, re.findall() returns an array of tuples,

with each tuple containing text matched by all the capturing groups.

The overall regex match is not included in the tuple, unless you place the entire regex

inside a capturing group.

More efficient than re.findall() is re.finditer(regex, subject). It returns an iterator that enables you to loop over the regex matches in the subject string: for m in re.finditer(regex, subject). The for-loop variable m is a Match object with the details of the current match.

How is Regex used in Splunk?

Field extraction

| rex field=_raw “%UC_CALLMANAGER-(?<Severity>\d+)-EndPointUnregistered:

Configure Line Breaking

LINE_BREAKER = [\r\n]+

Filtering and Routing Data to Queues

REGEX =(?m)^EventCode=(592|593)

Many more…….

Regex Testing Tools

• RegExr http://gskinner.com/RegExr/

• Reggy http://reggyapp.com/

• RegexPal http://regexpal.com/

• Regex Buddy http://www.regexbuddy.com/

• Lars Olav Torvik http://regex.larsolavtorvik.com/

• Rubular http://rubular.com/

Regex Reference Texts

• http://www.regular-expressions.info/reference.html - from the creators of RegexBuddy

• Introducing Regular Expressions by Michael Fitzgerald

• Mastering Regular Expressions by Jeffrey Friedl

• Regular Expressions Cookbook by Jan Goyvaerts

• Regular Expressions Pocket Reference by Tony Stubblebine

Basic Concepts of Regular Expressions

Because Knowingleads to Doing

Simple Pattern Matching

Matching String Literals

Matching Digits and Non-Digits

Matching Word and Non-Word Characters

Matching Whitespace

Matching Any Character

Matching String LiteralsSample Apache Log

10.23.10.11 www.iamcool.com 10.100.0.11 - - [06/Dec/2012:14:39:03 -0800] "GET /Facelift/answers/swelling HTTP/1.1" 301 20 14932 "-" "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)”

Literal String Match of the first ip address would be:

10.23.10.11

Matching Digits and Non-Digits\d or \D or [0-9]

\d - match digit

\D – match non-digit (matches whitespace, punctuation and other characters not used in words)

[0-9] - match any number (called a character class)

[^0-9] – match any non-number

Matching Words and Non-Words

\w or \W

\w – match any word character and is essentially the same as the character class [a-zA-Z0-9]

\W – match any non-word character

Matching Whitespace

\s or \S

\s – match whitespace (Spaces, Tabs, Line Feeds and Carriage Returns)

\S – match any character that is not whitespace. Same as [^\s]

Character shorthands for whitespace

Character Shorthand Description

\f Form Feed

\h Horizontal Whitespace

\H Not Horizontal Whitespace

\n Newline

\r Carriage Return

\t Horizontal Tab

\v Vertical Tab (whitespace)

\V Not vertical whitespace

Matching Any Character

Dot (.)

Matches any character but line ending characters

\b – matches a word boundary without consuming any characters

Boundaries and Alternation Matching the Beginning and End of Line

List of Regex Special Character

Alternation and Regex Options

Subpatterns

Capturing and Named Groups

Character Classes

Negated Character Classes22

Matching Beginning and End of Line

^ OR $

^ - matches the beginning of a line

$ - matches the end of a line

List of Regex Special Characters.^*+?|(){}[]\-

. -matches any character

^ -matches beginning of the line

* -matches zero or more

+ -matches one or more

? –matches one or more

| -used for alternation (choice of patterns to match)

() –used for grouping

{} –used as a quantifier

[] –used with character classes

\ -used to make a character literal or as a special regex character

- -hyphen is used in a character class range

Alternation and Options

| OR ?

| -gives choice of alternate patterns to match, ie: (THE|The|the)

(?i) – Case insensitive

(?J) –allow duplicate names

(?m) –match on duplicate lines

(?s) –match on a single line

(?U) –match lazy

(?X) –Ignore whitespace, comments

(?-…) –Unset or turn off options

Subpatterns

Group(s) within a group

(THE|The|the) -has three subpatterns

(tT)h(e|eir) –matches the, The, their, Their

Capturing and Named Groups

()(?<name>…) OR (?P<name>…)

Store their content in memory

(it is) (time to eat)

(?<Severity>\d)

Splunk creates a field of Severity from this named group

Character Classes

[aeiou] –only matches the characters inside of the brackets

[0-9] –matches a range of characters, using a hyphen

[a-zA-Z0-9] –matches all alphanumeric characters

Negated Character Classes

[^…]

*** Super important – especially for Splunk field extractions ***

[^aeiou] –matches all consonants and NOT vowels

[^\s] – match everything that is not a space

Quantifiers Greedy, Lazy, Possessive

Matching a certain number of times

Greedy, Lazy, Possessive* + ?

* - match zero of more times

.* -will match all of the characters in the subject text (want to avoid this)

+ -match one or more

\d+ -match all of the digits until there aren’t any more - greedy

? –match 0 or 1 of the preceeding token.

colou?r –matches either color or colour

Matching a Certain Number of Times{}

\d{3} -matches 3 digits only

\d{1,3} –matches range of 1 to 3 digits

\d{1,} -same as \d+

\d{0,} -same as \d*

\d{0,1} -same as \d?

Any Thoughts, Ideas, Feedback?splunkyoda@splunk.com

Optimized Regular Expressions

Because fast is elegant!

Optimize Regular ExpressionsGood Better

(whiskey) (?:whiskey)

Capture groups add unnecessary overhead and impact overallperformance use them only when necessary.

splunk|splash spl(?:unk|ash)

Try to “factor” on the left, when you can, while exposing requiredtext. Less alternation is better.

(?:aussie$|gypsie$) (?:aus|gyp)sie$

Try to “factor” on the right when input text is close to end of theline. Most regex engines will anchor at end of line when “$” is

present.

0{3,7} 0000{0,4}

Typically exposing required or literal text makes the engineexecute the regex faster

(.)* .*

Useless parenthesis add unnecessary overhead. As above, usethem only when necessary.

matty[:] matty:

The character class/set (indicated by []) will add unnecessaryoverhead when not needed.

^genti|^collar ^(?:genti|collar)

Anchoring the regex at the beginning of the line will result inimproved performance with most regex engines.

delaney$|connery$ (delaney|connery)$

I said, anchor the regex!

^src.*: ^src[^:]*:

Using a negated character class/set instead of lazy/greedyquantifiers will typically result in faster regexes. Lazy/greedy

quantifiers will make the regex engines backtrack whichultimately impacts overall performance.

bride|brian bri(?:de|an)

Full alternation is more expensive than partial alternation. Also,in this case the regex engine will alternate only AFTER ‘bri’ has

been matched.

(?:edu|com|net|…) (?:com|edu|net|…)

Leading the engine to a match by placing the most popular matchfirst may result in faster execution in some engines.

^.*(answer) ^.{42}(answer)

Specifying an exact position inside the string and leading theengine to a match, will help improve performance drastically

compared to using a simple greedy/lazy quantifier.

.*?a ^.*a

If ‘a’ is near the end of the input string will match faster as lessbacktracking will be required.

.*a ^.*?a

If ‘a’ is near the beginning of the input string the regex enginewill match faster.

:[^:]*: :[^:]*+:

Ex. in ‘ :destination’ the second regex fails faster.

:[^:]*: :(?>[^:]*):

Same as above, using different notation. Explanation:Atomic grouping or possessive quantifiers instruct the regexengine not to keep the states captured by * or + therefore

preventing it from unsuccessfully backtracking and in turn failingfaster.

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