Ruby

Regular Expression Catastrophic Backtrack

See more Regular Expressions Examples

This example demonstrates how adding a processing time limit prevents a catastrophic backtrack.

Catastrophic backtracking in regular expressions occurs when a poorly constructed pattern causes the regex engine to try an exponential number of possibilities, especially on non-matching input. This leads to extremely slow performance or even a program hang.

Example:

(a+)+$

Applied to:

aaaaaaaaaaaaaaaaaaaaaab

The regex engine tries many combinations of grouping a+ inside another +, looking for a way to match the whole string, but it never matches due to the final b. The nested quantifiers (+ inside +) are what trigger the backtracking explosion.

How to prevent it:

• Avoid nested quantifiers like (a+)+
• Use atomic groups or possessive quantifiers if available
• Consider more efficient regex design or a parser

Catastrophic backtracking is especially dangerous when regex patterns are applied to user-controlled input.

Ruby

require 'chilkat'

sbSubject = Chilkat::CkStringBuilder.new()

# Create data that would cause a catastrophic backtrack with the regular expression "((a+)+$)"
i = 0
while i < 500
    sbSubject.Append("aaaaaaaaaaaaaaaaaaaa")
    i = i + 1
end
sbSubject.Append("X")

pattern = "((a+)+$)"

json = Chilkat::CkJsonObject.new()
json.put_EmitCompact(false)

# Set a time limit to prevent a catastrophic backtrack..
# (Approx) 1 second time limit.
# This should fail:
numMatches = sbSubject.RegexMatch(pattern,json,1000)
if (numMatches < 1)
    print sbSubject.lastErrorText() + "\n";

    # 	We should get an error such as the following:

    # 	ChilkatLog:
    # 	  RegexMatch:
    # 	    ChilkatVersion: 11.1.0
    # 	    regex_match:
    # 	      timeoutMs: 1000
    # 	      Exceeded regular expression match limit.
    # 	      elapsedMs: Elapsed time: 797 millisec
    # 	      num_matches: -1
    # 	    --regex_match
    # 	  --RegexMatch
    # 	--ChilkatLog

    exit
end

# We shouldn't get here.
# The above data and regular expression should've caused a catastrophic backtrack.
print "numMatches: " + numMatches.to_s() + "\n";
print json.emit() + "\n";