Regular Expression using `expl3`

Question

Can anyone explain these regular expressions using expl3 and how those works?

\regex_replace_all:nnN { a\-(ai) } { \1 } \l_mytrans_tl
\regex_replace_all:nnN { a\-(au) } { \1 } \l_mytrans_tl
\regex_replace_all:nnN { a\-([āiueoīū]{1}) } { \1 } \l_mytrans_tl
\regex_replace_all:nnN { a\-(rr) } { \1 } \l_mytrans_tl
\regex_replace_all:nnN { a\-(r) } { \1 } \l_mytrans_tl
\regex_replace_all:nnN { \-([āiueoīū]{1}) } { \1 } \l_mytrans_tl

Especially the last one. I could not figure out this.

Answer 1

The last line should replace -ā by ā, -i by i, -u by u and so on.

To break it up:

• \- represents the character -.
• Next, everything wrapped in parenthesis (...) is the part of the string that should be use for the replacement.
• Then follows a group wrapped in square brackets [...], which essentially means "one of these".
• Following a repetition marker {1} meaning the character or group before "exactly once".
• As for the replacement, \1 selects the first selection of the string, that is, the first part wrapped in parenthesis (...), which is [āiueoīū]{1} in this case.

So, it means replace a - followed by one of āiueoīū, but only exactly one character of these, by this very character. Essentially, it removes the -.

See the documentation of the l3regex package which is currently included in the doc of the LaTeX3 interfaces (chapter 8).