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  1. #!/bin/bash
  2. s=' Size: 123'
  3. grep '^[[:blank:]]+ Size: [0-9]+' <<< "$s" # Bohemian
  4. echo "---"
  5. grep -E '^[ \t]+Size: [0-9]+' <<< "$s" # OP with -E
Success #stdin #stdout 0s 4956KB
stdin
Standard input is empty
stdout
---
  Size: 123