| b | rest of RE is a BRE | b | (not portable) |
| c | case-sensitive matching (overrides operator type) | c | (not portable) |
| e | rest of RE is an ERE | e | (not portable) |
| i | case-insensitive matching (overrides operator type) | i | ignorecase |
| m | historical synonym for n | m | (not portable) |
| n | newline-sensitive matching | n | (not portable) |
| p | partial newline-sensitive matching | p | (not portable) |
| q | rest of RE is a literal (“quoted”) string, all ordinary characters | q | (not portable) |
| s | non-newline-sensitive matching (default) | s | (not portable) |
| t | tight syntax | t | (not portable) |
| w | inverse partial newline-sensitive (“weird”) matching | w | (not portable) |
| x | expanded syntax | x | (not portable) |