regex to match entire words containing only certain characters

Question

I want to match entire words (or strings really) that containing only defined characters.

For example if the letters are d, o, g

Answer 1

Which regex flavor/tool are you using? (e.g. JavaScript, .NET, Notepad++, etc.) If it's one that supports lookahead and lookbehind, you can do this:

(?<!\S)[dog]+(?!\S)

This way, you'll only get matches that are either at the beginning of the string or preceded by whitespace, or at the end of the string or followed by whitespace. If you can't use lookbehind (for example, if you're using JavaScript) you can spell out the leading condition:

(?:^|\s)([dog]+)(?!\S)

In this case you would retrieve the matched word from group #1. But don't take the next step and try to replace the lookahead with (?:$|\s). If you did that, the first hit ("dog") would consume the trailing space, and the regex wouldn't be able to use it to match the next word ("god").

Answer 2

This should work for you

\b[dog]+\b(?![,])

Explanation

r"""
\b        # Assert position at a word boundary
[dog]     # Match a single character present in the list “dog”
   +         # Between one and unlimited times, as many times as possible, giving back as needed (greedy)
\b        # Assert position at a word boundary
(?!       # Assert that it is impossible to match the regex below starting at this position (negative lookahead)
   [,]       # Match the character “,”
)
"""

Answer 3

The following regex represents one or more occurrences of the three characters you're looking for:

[dog]+

Explanation:

The square brackets mean: "any of the enclosed characters".

The plus sign means: "one or more occurrences of the previous expression"

This would be the exact same thing:

[ogd]+

Answer 4

Depending on the language, this should do what you need it to do. It will only match what you said above;

this regex:

[dog]+(?![\w,])

in a string of ..

dog god ogd, dogs o

will only match..

dog, god, and o

Example in javascript

Example in php

Anything between two [](brackets) is a character class.. it will match any character between the brackets. You can also use ranges.. [0-9], [a-z], etc, but it will only match 1 character. The + and * are quantifiers.. the + searches for 1 or more characters, while the * searches for zero or more characters. You can specify an explicit character range with curly brackets({}), putting a digit or multiple digits in-between: {2} will match only 2 characters, while {1,3} will match 1 or 3.

Anything between () parenthesis can be used for callbacks, say you want to return or use the values returned as replacements in the string. The ?! is a negative lookahead, it won't match the character class after it, in order to ensure that strings with the characters are not matched when the characters are present.