Programming
Memory App
Regex



Quantifiers






Plus (+) and star (*)

Similar to the ? (question mark) are + (plus) and * (star). Metacharacter + means one or more of the immediately-preceding item. Metacharacter * means any (none or more) the immediately-preceding item.

Example <H1>

HTML specifications says that spaces are allowed before the closing >. Regex <H[1-6] *> matches <H1>, but also <H1 > or <H1 > (with any number of spaces).

Example <HR SIZE=14>

Regex <HR +SIZE *= *[0-9]+ *> matches <HR SIZE=1>, but also <HR SIZE = 14 > (with any number of spaces). Our eyes has always been trained to treat spaces specially. That's a habit you will have to break when reading regex. The space character is a normal character, no different from say, j or 4.

Example (SIZE optional)

If we want SIZE to be optional the regex will be /<HR( +SIZE *= *[0-9]+)? *>/ Note that the ending " *" is kept outside group parantheses. This still allows something such as <HR >. Note that the first " +" is included withing the parantheses. Otherwise, a space would have been required after the HR. This would cause <HR> not to match.

Interval quantifier {min, max}

You might use [a-zA-Z]{1,5} to match a US stock ticker (from one to five letters). Notation {0,1} is the same as ? (question mark). If only a single number is given (such as [a-z]{3}), it matches exactly that many of the item. regexpal.com/?fam=111476
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