Xa2multi Pl

xa2multi-pl

Tiny Perl filter for BWA SAM output. For every alignment line containing XA:Z:..., it prints the original line unchanged and appends one extra SAM record per alternate hit, marking those records as secondary alignments and copying the mismatch count into NM:i.

Quick Start

• Command: xa2multi.pl [input.sam]
• Local executable: /home/vimalinx/miniforge3/envs/bio/bin/xa2multi.pl
• Input style: SAM on stdin or as filenames on the command line

When To Use This Tool

• Converting BWA alternate hits from compact XA:Z tags into explicit SAM records
• Preparing alignments for downstream tools that ignore or discard XA optional fields
• Inspecting or counting alternate mappings as ordinary SAM records
• Normalizing legacy BWA output before later SAM/BAM processing

Common Patterns

# Expand XA-tagged alignments from a SAM file
xa2multi.pl input.sam > expanded.sam

# Stream from stdin into another SAM/BAM step
samtools view -h input.bam | xa2multi.pl | samtools view -b -o expanded.bam

Observed in local testing:
primary line: unchanged
alternate hits: emitted with flags 256 / 272 and NM:i copied from the XA field

Recommended Workflow

1. Start from BWA SAM output that actually contains XA:Z tags.
2. Run xa2multi.pl before downstream tools that expect each alignment as its own SAM record.
3. Redirect stdout to a new SAM/BAM stream because the script never edits files in place.
4. Validate a few records afterward to confirm the expected number of alternate alignments were emitted.

Guardrails

• The script has no built-in -h or --version; it simply reads stdin or filenames via Perl's <> operator.
• It only reacts to lines containing XA:Z:. All other lines, including SAM headers, are printed unchanged.
• Source inspection shows each alternate alignment is emitted as a secondary record with 0x100 set, MAPQ forced to 0, and TLEN forced to 0.
• If the alternate orientation differs from the primary alignment, the script reverse-complements the sequence and reverses the quality string.
• The source includes # FIXME: TLEN/ISIZE is not calculated!, so do not treat inferred template length as trustworthy after expansion.