How does the regular expression ‘(?

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They are called lookarounds; they allow you to assert if a pattern matches or not, without actually making the match. There are 4 basic lookarounds:

  • Positive lookarounds: see if we CAN match the pattern…
    • (?=pattern) – … to the right of current position (look ahead)
    • (?<=pattern) – … to the left of current position (look behind)
  • Negative lookarounds – see if we can NOT match the pattern
    • (?!pattern) – … to the right
    • (?<!pattern) – … to the left

As an easy reminder, for a lookaround:

  • = is positive, ! is negative
  • < is look behind, otherwise it’s look ahead

References

But why use lookarounds?

One might argue that lookarounds in the pattern above aren’t necessary, and #([^#]+)# will do the job just fine (extracting the string captured by \1 to get the non-#).

Not quite. The difference is that since a lookaround doesn’t match the #, it can be “used” again by the next attempt to find a match. Simplistically speaking, lookarounds allow “matches” to overlap.

Consider the following input string:

and #one# and #two# and #three#four#

Now, #([a-z]+)# will give the following matches (as seen on rubular.com):

and #one# and #two# and #three#four#
    \___/     \___/     \_____/

Compare this with (?<=#)[a-z]+(?=#), which matches:

and #one# and #two# and #three#four#
     \_/       \_/       \___/ \__/

Unfortunately this can’t be demonstrated on rubular.com, since it doesn’t support lookbehind. However, it does support lookahead, so we can do something similar with #([a-z]+)(?=#), which matches (as seen on rubular.com):

and #one# and #two# and #three#four#
    \__/      \__/      \____/\___/

References

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