- Capturing optional expressions.
/a?typical/ matches “typical” and “atypical”- If an optional character is inside parenthesis, zero-wdith matches are still captured.
/(A?)B/ matches “AB” and captures “A”/(A?)B/ matches “B” and captures “”./(a?)typical/ matches “typical” and captures “”.
- Backreference to a zero-width capture is also zero-width.
- An example expression is
/(a?)typical & \1political/ matches “atypical & apolitical”.
- Can make Group Expressions optional.
/(un)?willing/ matches “willing” and “unwilling”- An optional group is only captured if it matches.
- Another example is
/(A)?B/ matches “AB” and captures “A”.
/(A)?B/ matches “B” but captures nothing./(un)?willing/ matches “willing” but captures nothing.
- An optional group that did not match, will not be captured. It has no backreference.
/(un)?willing & \1able/ matches “unwilling & unable”
- To not match the above, we can use
/(un)?willing & \1able/ does not match “willing & able”
- The above is true in every regex engine apart from JavaScript.
- JavaScript’s regex engine captures optional groups. Can make backreferences to them.
- Solution: capture the optional group.
/((un)?)willing & \1able/ matches “unwilling & unable”./((un)?)willing & \1able/ matches “willing & able”.
(un)? matches, captured by inner parenthesis, assigned to \1.(un)? does not match, captured by outer parenthese, assigned to \1.