Question

I need to write the Lua equivalent of the following regular expression (regex)

\b[0-9]*.\b[0-9]*(?!])

for use with Lua’s string.gmatch. Can this be done?

For reference, the above expression matches both the integer and fractional part of a number containing a decimal separator (e.g. 1, 1.1, 0.1, 0.11 all match fully). However, if a trailing ] is present, then only the integer part is matched (e.g. only 1. is matched in 1.1]).

Answer 1

Lua does not have regular expressions, mainly because a full regular expression library would be bigger than Lua itself.

What Lua has instead are matching patterns, which are way less powerful (but still sufficient for many use cases):

• There is no “word boundary” matcher,
• no alternatives,
• and also no lookahead or similar.

I think there is no Lua pattern which would match every possible occurrence of your string, and no other one, which means that you somehow must work around this.

The pattern proposed by Stuart, %d*%.?%d*, matches all decimal numbers (with or without a dot), but it also matches the empty string, which is not quite useful. %d+%.?%d* matches all decimal numbers with at least one digit before the dot (or without a dot), %d*%d.?%d+ matches all decimal numbers with at least one digit after the dot (or without a dot). %.%d+ matches decimal numbers without a digit before the dot.

A simple solution would be to search more than one of these patterns (for example, both %d+%.?%d* and %.%d+), and combine the results. Then look at the places where you found them and look if there is a ‘]’ following them.

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I experimented a bit with the frontier pattern.

The pattern %f[%.%d]%d*%.?%d*%f[^%.%d%]] matches all decimal numbers which are preceded by something that is neither digit nor dot (or by nothing), and followed by something that is neither ] nor digit nor dot (or by nothing). It also matches the single dot, though.