4 SC2063

Joachim Ansorg edited this page 2021-11-12 19:26:56 +01:00

Table of Contents

• Grep uses regex, but this looks like a glob.
• Problematic code:
• Correct code:
• Rationale:
• Exceptions

Grep uses regex, but this looks like a glob.

Problematic code:

grep '*foo*'

Correct code:

grep 'foo'   # or more explicitly,  grep '.*foo.*'

Rationale:

In globs, * matches any number of any character.

In regex, * matches any number of the preceding character.

grep uses regex, not globs, so this means that grep '*foo' is nonsensical because there's no preceding character for *.

If the intention was to match "any number of characters followed by foo", use '.*foo'. Also note that since grep matches substrings, this will match "fishfood". Use anchors to prevent this, e.g. foo$.

This also means that f* will match "hello", because f* matches 0 (or more) "f"s and there are indeed 0 "f" characters in "hello". Again, use grep 'f' to find strings containing "f", or grep '^f' to find strings starting with "f".

Exceptions

If you're aware of the differences between globs and regex, you can ignore this message.

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