What

isLineContinuation (in the requirements.txt parser) ran a backtracking regexp on every line of every requirements*.txt file:

re := cachedregexp.MustCompile(`([^\\]|^)(\\{2})*\\$`)
return re.MatchString(line)

The check is just "does the line end in an odd number of backslashes?". This PR replaces the regexp with a trailing-backslash count done iteratively without regexes.

trailingBackslashes := 0
for i := len(line) - 1; i >= 0 && line[i] == '\\'; i-- {
    trailingBackslashes++
}
return trailingBackslashes%2 == 1

Why

I CPU-profiled a full scan of the dd-source monorepo (large, Python-heavy). pprof -peek showed regexp.(*Regexp).MatchString was driven 100% by python.isLineContinuation, and it accounted for ~3.9s cumulative — ~16% of total scan CPU, almost entirely regexp backtracking.

Before — top self-time, dd-source scan (24.2s CPU total):

   flat  flat%        cum   cum%
 15.93s 65.77%     15.93s 65.77%  syscall.rawsyscalln
  1.93s  7.97%      2.01s  8.30%  path/filepath.matchChunk
  1.77s  7.31%      3.82s 15.77%  regexp.(*Regexp).tryBacktrack      <- isLineContinuation
  0.82s  3.39%      0.82s  3.39%  regexp/syntax.(*Inst).MatchRunePos <- isLineContinuation
  0.57s  2.35%      0.57s  2.35%  regexp.(*bitState).shouldVisit     <- isLineContinuation
  0.31s  1.28%      0.36s  1.49%  regexp.(*inputString).step         <- isLineContinuation
  0.16s  0.66%      0.17s  0.70%  regexp.(*bitState).push            <- isLineContinuation

After — same scan (19.1s CPU total): every regexp.* frame is gone.

   flat  flat%        cum   cum%
 14.94s 78.06%     14.94s 78.06%  syscall.rawsyscalln
  2.25s 11.76%      2.32s 12.12%  path/filepath.matchChunk
  0.44s  2.30%      0.44s  2.30%  path/filepath.scanChunk
  0.22s  1.15%      2.98s 15.57%  path/filepath.Match
  0.03s  0.16%      2.71s 14.16%  go-git/.../gitignore.(*pattern).simpleNameMatch

Results

What	graph
dd-source scan cpu profile before
dd-source scan cpu profile after

Correctness

The trailing-backslash count is behavior-identical to ([^\\]|^)(\\{2})*\\$ across all inputs (empty string, leading backslash, and odd/even runs of trailing backslashes). The existing pkg/extractor/python/fixtures/pip/line-continuation.txt fixture test passes unchanged; go vet and the full pkg/extractor/python suite are green.