How does the ? make a quantifier lazy in regex

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? can mean a lot of different things in different contexts.

  • Following a normal regex token (a character, a shorthand, a character class, a group…), it means “Match the previous item 0-1 times”.
  • Following a quantifier like ?, *, +, {n,m}, it takes on a different meaning: “Make the previous quantifier lazy instead of greedy (if that’s the default; that can be changed, though – for example in PHP, the /U modifier makes all quantifiers lazy by default, so the additional ? makes them greedy).

  • Right after an opening parenthesis, it marks the start of a special construct like for example

    a) (?s): mode modifiers (“turn on dotall mode”)
    b) (?:...): make the group non-capturing
    c) (?=...) or (?!...): lookahead assertion
    d) (?<=...) or (?<!...): lookbehind assertion
    e) (?>...): atomic group
    f) (?<foo>...): named capturing group
    g) (?#comment): inline comments, ignored by the regex engine
    h) (?(?=if)then|else): conditionals

and others. Not all constructs are available in all regex flavors.

  • Within a character class ([?]), it simply matches a verbatim ?.

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