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Table Of Contents
Regex Engine Performance Enhancement
Prerequisites for Regex Engine Performance Enhancement
Information About Regex Engine Performance Enhancement
Default Regular Expression Engine
New Regular Expression Engine Selection
How to Change the Regular Expression Engine
Selecting the New Regular Expression Engine
Regex Engine Performance Enhancement
The Regex Engine Performance Enhancement feature introduces a new regular expression engine that is designed to process complex regular expressions. This new regular expression engine does not replace the existing engine. The existing engine is preferred for simple regular expressions and is the default engine and in Cisco IOS software. Either engine can be selected from the command-line interface (CLI).
Feature History for the Regex Engine Performance Enhancement Feature
Finding Support Information for Platforms and Cisco IOS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.
Contents
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Prerequisites for Regex Engine Performance Enhancement
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Prerequisites for Regex Engine Performance Enhancement
The regular expression engine can be selected only under a Border Gateway Protocol (BGP) routing process in router configuration mode. So, the engine can be changed only after BGP has been enabled.
Information About Regex Engine Performance Enhancement
To select a regular expression engine in Cisco IOS software, you must understand the following concepts:
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Regular Expression Overview
A regular expression is a pattern to match against an input string. You specify the pattern that a string must match when you compose a regular expression. Matching a string to the specified pattern is called "pattern matching." Pattern matching either succeeds or fails.
A regular expression can be a single-character pattern or a multiple-character pattern. That is, a regular expression can be a single character that matches the same single character in the input string or multiple characters that match the same multiple characters in the input string.
Default Regular Expression Engine
The default Cisco IOS regular expression engine uses a recursive algorithm. This engine is effective but uses more system resources as the complexity of regular expressions increase. The recursive algorithm works well for simple regular expressions, but is less efficient when processing very complex regular expressions because of the backtracking that is required by the default engine to process partial matches. In some cases, CPU watchdog timeouts and stack overflow traces have occurred because of the length of time that the default engine requires to process very complex regular expressions.
New Regular Expression Engine Selection
The Regex Engine Performance Enhancement feature introduces a deterministic processing time regular expression engine in Cisco IOS software. This new engine does not replace the default regular expression engine. The new engine employs an improved algorithm that eliminates excessive back tracking and greatly improves performance when processing complex regular expressions. When the new engine is enabled, complex regular expressions are evaluated more quickly, and CPU watchdog timeouts and stack overflow traces will not occur. However, the new regular expression engine takes longer to process simple regular expressions than the default engine.
We recommend that you use the new regular expression engine if you need to evaluate complex regular expressions or if you have observed problems related to evaluating regular expressions. We recommend that you use the default regular expression engine if you use only simple regular expressions. The new engine can be enabled by entering the bgp regexp deterministic command under a BGP routing process. The default regular expression engine can be reenabled by entering the no form of this command.
How to Change the Regular Expression Engine
Selecting the New Regular Expression Engine
We recommend that you use the new regular expression engine if you need to evaluate complex regular expressions or if you have observed problems related to evaluating regular expressions. We recommend that you use the default regular expression engine if you only use simple regular expressions.
Prerequisites
The regular expression engine can be selected only under a BGP routing process in router configuration mode. So, the engine can be changed only after BGP has been enabled.
SUMMARY STEPS
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DETAILED STEPS
Examples
The following example configures Cisco IOS software to use the default regular expression engine:
router bgp 1no bgp regexp deterministicThe following example configures Cisco IOS software to use the deterministic processing time regular expression engine:
router bgp 1bgp regexp deterministicAdditional References
The following sections provide references related to the Regex Engine Performance Enhancement feature.
Related Documents
Regular Expressions
"Regular Expressions" appendix of the Cisco IOS Terminal Services Configuration Guide
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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MIBs
RFCs
No new or modified RFCs are supported by this feature, and support for existing standards has not been modified by this feature.
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Technical Assistance
Technical Assistance Center (TAC) home page, containing 30,000 pages of searchable technical content, including links to products, technologies, solutions, technical tips, tools, and lots more. Registered Cisco.com users can log in from this page to access even more content.
TAC Home Page:
http://www.cisco.com/public/support/tac/home.shtml
BGP Support Page:
http://www.cisco.com/cgi-bin/Support/browse/psp_view.pl?p=Internetworking:BGP
Command Reference
This section documents a new command. All other commands used with this feature are documented in the Cisco IOS command reference publications.
bgp regexp deterministic
To configure Cisco IOS software to use the deterministic processing time regular expression engine, use the bgp regexp deterministic command in router configuration mode. To configure Cisco IOS software to use the default regular expression engine, use the no form of this command.
bgp regexp deterministic
no bgp regexp deterministic
Syntax Description
This command has no keywords or arguments.
Defaults
The default regular expression engine is enabled.
Command Modes
Router configuration
Command History
Usage Guidelines
The default Cisco IOS regular expression engine uses a recursive algorithm. This engine is effective but uses more system resources as the complexity of regular expressions increases. The recursive algorithm works well for simple regular expressions, but is less efficient when processing very complex regular expressions because of the backtracking that is required by the default engine to process partial matches. In some cases, CPU watchdog timeouts and stack overflow traces have occurred because of the length of time that the default engine requires to process very complex regular expressions.
The deterministic processing time regular expression engine does not replace the default regular expression engine. The new engine employs an improved algorithm that eliminates excessive backtracking and greatly improves performance when processing complex regular expressions. When the new engine is enabled, complex regular expressions are evaluated more quickly, and CPU watchdog timeouts and stack overflow traces will not occur. However, the new regular expression engine takes longer to process simple regular expressions than the default engine.
We recommend that you use the new regular expression engine if you need to evaluate complex regular expressions or if you have observed problems related to evaluating regular expressions. We recommend that you use the default regular expression engine if you use only simple regular expressions. The new engine can be enabled by entering the bgp regexp deterministic command under a Border Gateway Protocol (BGP) routing process. The default regular expression engine can be reenabled by entering the no form of this command.
Examples
The following example configures Cisco IOS software to use the deterministic processing time regular expression engine:
Router(config)# router bgp 1Router(config-router)# bgp regexp deterministicThe following example configures Cisco IOS software to use the default regular expression engine:
Router(config)# router bgp 1Router(config-router)# no bgp regexp deterministic
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