Regex Notes

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Regular expressions commonly referred to as regex allow us to extract information from any text by searching for one or more matches of a specific pattern or sequence. For example:

Anchors - ^and$

Quantifiers - * + ? and {}

OR operator - | or []

Character classes -\d \s and

In order for special characters to be taken literally, the characters ^.[$()|*+?{\ must be escaped with a backslash.

Flags

A regex usually come within the form /abc/, where the search pattern is delimited by two slash characters /. At the end we can specify a flag with these values(we can also combine them with each other):

• g (global) does not return after the first match, restarting the subsequent searches from the end of the previous match.

• m (multi-line) when enabled ^ and $ will match the start and end of a line, instead of the whole string.

• i (insensitive) makes the whole expression case-insensitive (for instance /aBc/i would match AbC)

Grouping - ()

If we choose to put a name to the groups (using (?<foo>..)) we will be able to retrieve the group values using the match result like a dictionary where the keys will be the name of each group.

Bracket expressions - []

Greedy and Lazy Match

The quantifiers (* + {}) are greedy operators, so they expand the match as far as they can through the provided test.

For example, <.+> matches <div>simple</div> in This is a <div>simple div</div> test. In order to catch only the div tag we can use a ? to make it lazy:

Boundaries - \b and \B

\b represents an anchor like caret matching positions where one side is a word characters and the other sie is not a word character (for instance it may be the beginning of the string or a space character).

\babc\b performs a whole words only search

\B matches all positions where \b doesn’t match and could be if we want to find a search pattern fully surrounded by word characters.

\Babc\B matches only if the pattern is fully surrounded by word characters.

Look-ahead and Look-Behind - (?=) and (?<=>)

d(?=r) matches a d only if it is followed by r, but r will not be part of the overall match (?~r)d matches a d only if it is preceded by an r, but r will not be part of the overall match

You’re also able to use the negation operator

d(?!r) matches a d only if it is not follow by r, but r will not be part of the overall match (?<!r)d matchs a d only if it is not preceded by r, but r will not be part of the overall match.

Source

Jonny Fox - Regex tutorial - A quick cheatsheet by examples