# sed, regex, and invalid references

## 2015/06/10

sed is a handy tool for line-by-line processing. Unfortunately it’s a little older, so its regex syntax is just different enough from Perl/Python/Ruby to get me confused.

Using sed without any flags uses old-style regex. The thing that gets me here is to you need to escape + if you want to capture at least one repetition of the preceding expression. That means that /[0-9]+/ will match 1+ but /[0-9]+/ will match 1234.

You also need to escape backreference parentheses, so if you want a number 1 to turn into a number "1" you need s_([0-9])_"1"_, i.e., you need ( and ).

Using the advanced regex flag -r means that you the escaping takes on the opposite meaning, so that /[0-9]+/ now matches 1234 while [0-9]+ matches 1+, and the substitution above is s_([0-9])_"1"_, i.e., now with unescaped parentheses.