What do brackets mean in regex

When invalid is not used, violations of the specified syntax or semantics for REs produce undefined results: this may entail an error, enabling an extended syntax for that RE, or using the construct in error as literal characters to be matched.

The use of regular expressions is generally associated with text processing. If during the operation of a standard utility a NUL is included in the text designated to be matched, that NUL may designate the end of the text string for the purposes of matching. When a standard utility or function that uses regular expressions specifies that pattern matching shall be performed without regard to the case uppercase or lowercase of either data or patterns, then when each character in the string is matched against the pattern, not only the character, but also its case counterpart if any , shall be matched.

This definition of case-insensitive processing is intended to allow matching of multi-character collating elements as well as characters, as each character in the string is matched using both its cases. For example, in a locale where "Ch" is a multi-character collating element and where a matching list expression matches such elements, the RE "[[. The implementation shall support any regular expression that does not exceed bytes in length. A bracket expression shall match a single character or a single collating element.

A BRE special character has special properties in certain contexts. The BRE special characters and the contexts in which they have their special meaning are as follows:. A bracket expression an expression enclosed in square brackets, "[]" is an RE that shall match a specific set of single characters, and may match a specific set of multi-character collating elements, based on the non-empty set of list expressions contained in the bracket expression.

A bracket expression is either a matching list expression or a non-matching list expression. It consists of one or more expressions: ordinary characters, collating elements, collating symbols, equivalence classes, character classes, or range expressions. Otherwise, it shall terminate the bracket expression, unless it appears in a collating symbol such as "[.

The special characters '. The character sequences "[. These symbols shall be followed by a valid expression and the matching terminating sequence ". A matching list expression specifies a list that shall match any single character that is matched by one of the expressions represented in the list.

An ordinary character in the list should only match that character, but may match any single character that collates equally with that character; for example, "[abc]" is an RE that should only match one of the characters 'a' , 'b' , or 'c'. It is unspecified whether a matching list expression matches a multi-character collating element that is matched by one of the expressions.

It is unspecified whether a non-matching list expression matches a multi-character collating element that is not matched by any of the expressions. Unihedron Unihedron I just found this out the hard way. Alan Moore Sheila Sheila 1 1 silver badge 2 2 bronze badges. Welcome to SO! Replacing or matching, it's just plain wrong. A lot of people make that mistake, and they usually get away with it--for years, sometimes--because their input strings never happen to contain a pipe.

Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. Podcast Explaining the semiconductor shortage, and how it might end.

Does ES6 make JavaScript frameworks obsolete? A very wide search, most likely not that useful all the time. So you might like the bracket-search you wrote earlier and want to re-use it for more than just one character. Then we can use quantifiers. The plus sign takes care of that empty string problem. But, what if you know the exact amount of times this bracket-match should occur?

Like when looking for Norwegian zip-codes always four digits. But, what if we want to find a year in a string. A year can be written with two or four digits. Can we do that with Regex? Of course! With the comma we can define more than one quantifier for that match. Thus covering almost any example of matching we can come up with.

So as you can see with brackets, the dot, and the quantifiers we can do pretty cool things. And this is still just the beginning. And yes, this is a big part of why Regex is a bit confusing to start up with. If used together they would mean that only what you search for is allowed in that string, nothing else.

But, you will get even more out of Regex when you start doing replacements in strings, and to do that you need to store your matches in memory. And you do this by wrapping parts of your Regex in parentheses. In Regex this is called grouping. Well I could go on explaining more and more about all things Regex that I've learnt from Rustam's presentation. First we have the square brackets, this means that we will start at something matching what's inside them.

If you do not need the group to capture its match, you can optimize this regular expression into Set? The question mark and the colon after the opening parenthesis are the syntax that creates a non-capturing group. The question mark after the opening parenthesis is unrelated to the question mark at the end of the regex.

The final question mark is the quantifier that makes the previous token optional. This quantifier cannot appear after an opening parenthesis, because there is nothing to be made optional at the start of a group. Therefore, there is no ambiguity between the question mark as an operator to make a token optional and the question mark as part of the syntax for non-capturing groups, even though this may be confusing at first.